Once two people are married they are prohibited by law from entering into contractual arrangements with each other, except in the context of divorce, with what is called a separation agreement.
Assume that because it is a second marriage which is anticipated, two people wish to be certain to provide for their respective children of their prior marriages. They want to be certain that upon their demise, their children inherit those assets which they considered their separate property when they entered into this second marriage.
A way to legally accomplish this is through use of an instrument called an antenuptial agreement, also called a pre-nuptial agreement.
Perhaps it is not right for you, but it is something which should always be considered before entering into a second marriage, or when entering into a first marriage when there is a wide disparity in the value of the assets of the two parties.