Key for stepfamilies: keeping the lines of communication open

Nowadays many people exchanging wedding vows are getting not only a spouse and three toasters, but a prefab family into the bargain. Stepfamilies are forming at the rate of 1,300 a day across the country, according to the Stepfamily Association of America, in Baltimore.

For these families trying to smooth the blend of his, hers, and theirs, communication is the key.

A prenuptial agreement may be in order if the couple have any significant property and assets. And the prospective bride and groom should sit down together and discuss in detail their expectations, including financial expectations, says Emily Visher, a Palo Alto, Calif., psychologist and author, with her husband, John, of ''How to Win as a Stepfamily'' (Dembner Books, New York, 196 pages, $13.95).

As to just how to pool resources, Dr. Visher says, ''There are many different ways of doing it, but the important thing is that people feel the way they're doing things is the best way for them.''

What is not a good idea is keep his and her funds completely separate, with each parent paying for his or her own children's needs, Dr. Visher says.

People going into second marriages may have a number of reasons to feel anxious about money in a way they didn't before their first marriage. But if there isn't some pooling of resources, there will be two families - often with separate standards of living - under one roof, and this situation could lead to another divorce.

''What's important is that all the children are treated somewhat fairly,'' Dr. Visher says. ''It's not that kids expect the same treatment from everyone; they know they don't feel the same way about someone they've just met as someone they've known for a long time. But if the grandparents give their own grandchildren a stereo, and the stepchildren just get a pair of socks or something, there will be some hurt feelings.''

Stepparents generally do not have a legal obligation to support stepchildren they have not adopted. But if a custodial parent remarries, the new spouse's financial resources may be taken into account if the noncustodial parent seeks modification of a court-ordered support arrangement, says Edward D. Tarlow, a lawyer with the Boston firm of Bradley, Barry & Tarlow.

And there are times when it is a good idea ''to pay up to make the problem go away,'' he adds.

For example, if Junior goes wild in the stereo shop and brings home $2,000 worth of nonreturnable sound equipment, noncustodial Dad might start yelling at ex-wife Mom and her new husband, ''What's the matter with you people? Can't you control that boy?'' Stepdad could end up digging into his pocket to help settle the bill.

Greg Confair, president of Sigma Financial Inc., in Whitehall, Pa., decries the ''punitive'' mentality of divorcing spouses, who sometimes refuse, for example, to sign a joint tax return.

If it's hard to keep the lines of communication open, he suggests contemplating the tax advantages that can be realized by doing so:

Suppose a tennis instructor is getting divorced from a neurosurgeon whom he put through school and from whom he expects alimony: $150,000 over 10 years, to be exact. Since the neurosurgeon is in a 50 percent tax bracket, it costs her only $75,000 in after-tax dollars to provide $150,000. Alimony is deductible for the one paying it and taxable income for the one receiving it. Since the tennis instructor is in a 20 percent tax bracket, he can get $150,000 in alimony with a tax bite of only $30,000.

''And so you end up with Uncle Sam chipping into the deal to the tune of $45, 000,'' says Mr. Confair.

This technique is useful when one divorcing spouse is in a significantly higher tax bracket than the other. If both are in high brackets, presumably no alimony is necessary. If both are on the humbler end of the scale, the recipient will want to have money from the former spouse characterized as child support, since that is not taxable. Child support is not deductible, either, however; so the contributing ex-spouse will prefer to have the money characterized as alimony.

All this explains why people hire lawyers.

Confair stresses another important point: If you already have a Clifford trust for your children's education in place, don't spell out in the divorce settlement either parent's responsibility for educating the children. Why? If a Clifford trust is used to satisfy a parent's specific legal obligation - as sending the children to college would be if specified in a divorce settlement - the tax advantages of the trust disappear. Clifford trust money going to send the kids to Yale would become ''imputed income,'' and taxable, for the parent designated to foot the tuition bills, and would be taxable.

Better just to leave the Clifford trust quietly in place, unmentioned during the divorce settlement.

Many planners recommend prenuptial agreements for second marriages, to specify responsibilities for the children and, in case of divorce, division of assets.

Christopher Croft, vice-president and manager of the financial advisory division of Bailard, Biehl & Kaiser, in San Mateo, Calif., recommends that remarriers with annual salaries in the $50,000 range consider such a step. The documentation costs about $200 to $1,000, but it's ''extremely critical'' to define who owns what assets - not only to make things easier in case of divorce, but for estate planning as well. ''There's no point in paying estate taxes on assets that aren't yours.''

Of course, preplanning a divorce settlement hardly sounds like a step on the path to marital bliss. But Mr. Confair argues that a premarital agreement gets people to think issues through - to ''coalesce their feelings.''

''It's better to confront the issues up front - in a more mature, responsible way,'' Mr. Croft argues. And if the relationship can't sustain that scrutiny, well. . . He suggests it's probably not on a strong basis.

Affluent parents who have remarried and want to provide for their widows but leave most of their estate to their children can set up a QTIP trust - a qualified terminable income property trust. Trust income can support a widow during her lifetime, with the principal reverting to the children after her death.