You get to pick the relationship each child has to the parents. The following screenshot shows the default relationship between a child and parents. Where it says “Birth” you can pick from a dropdown to pick “Foster” or “Stepchild”, or you can type something completely different in.
So for one family, you can list the relationship is “Birth” and another as “Possible Birth”. Or the relationship in both as “Birth”. Reports such as “Pedigree” will pick the first listed set of parents for reports. You can re-order to make the one you think is more likely first. I don’t think any reports really show multiple sets of parents. As I noted, Gramps isn’t really set up at the moment to do reports that are helpful for analysis. But you can do the data storage.
Phil.