Are you adding children manually or by merging from another tree or by finding the family in a later census with defined relationships?
If you are adding manually, the attached clip shows (at the bottom) how to select one or both parents. If you know that the person is a child of both, select both.
If you are merging from another tree (or several trees) you can often wind up with duplicates. Be very wary of adding in that manner.
If you are merging from the census, be wary, since the defined relationships may not be accurate (stepchildren, for example).
Hope this helps.