I know it isn't too big a deal, but wouldn't it be better to have only one site cover that?
Most of the times you are right. Cross-posting is not desired, since that brings separate answers to the same question, where the goal of SE is to have a single question/answer set.
But cross-posting isn't the case here. In the case of the same, or related question, usually from different users, there isn't always a problem as long as it is on-topic for the site being asked. Think of cases where answers between sites can have a different point of view. Questions might fit well on multiple sites.
If you intend to ask a question yourself, pick the site you think the question fits best. If it yields no result, you could delete and re-ask on the other site or if possible, flag the question and ask a mod to move your question.
Subject overlap isn't a problem.
But I'd see this as not even overlapping in all cases. The experts on biology will be able to identify things that GL experts might have a harder time with, and vice versa. They are two separate, though related, fields* and the reason for identification can be a very good reason to ask one group or the other.