Like Fiona said, adding the correct children to the correct parent match-up. You can go into the edit tab for any profile and add another spouse, without an issue. You just need to be careful while adding children of people with multiple spouses so that the right child is attributed to the right spouse.
For example, my great-grandfather Leonard (Utting-6) was married twice. He's listed as the husband of both Grace Press and Winifred Taylor, and all of his children are listed under. It is when you go into their individual profiles that you can see that his son Walter is his child by Grace, while his remaining children (including my grandfather, John) were born to Winifred.