My personal opinion on this (which is not that of the moderation team), is that we should leave them alone and just try to answer them. I really don't understand this insistance that someone should ask on our corner of the internet and nowhere else.
Now, I do agree that it would be very nice of the OP if they were to link the two and repeat any answer given on one on the other, but surely the main objectives here are i) to share knowledge and ii) to get answers to questions. Both of those objectives seem to me to be served perfectly well by cross posting.
Do we really want to close a question just because the OP didn't choose us over BioStars or vice versa? We're a very small site still and the volume of posts we get is tiny. If we tell people that in order to ask here they must ask here and nowhere else but here, the chances are we'd just drive them away to the more populous communities where they'll feel they have a greater chance of having their question answered.
If a question asked here already has a great answer on BioStars, we should just copy the answer here—giving credit to the original author, of course—and link back to the original answer. Even better, we can read the answer given, add our own two cents and post a new, better answer here.
But if we decide that any question that has been asked elsewhere cannot be asked here, we deprive the site of a wealth of useful information. I think it is important to build a good collection of interesting and helpful Q&As here, and removing good questions just because they have been asked elsewhere is directly against that goal.