help pleaseee. So conflicted with these two cute fur balls

so long story short me and my girlfriend found these two baby cats (4 to 5 weeks old) in our backyard storage unit. the dark one is a boy and the white one is a girl. We really want to keep the boy and want to give the white one to a shelter. The only problem is it seems like they are bonded in a way. They aren't always next to each other but the girl definitely finds her way back to her brother and loves being all over him. We already have two cats so adding two more would just be overwhelming for us. Would it be messed up to separate them? any advice would be greatly appreciated thank you