This post would be better served in the Filemaker Pro forum instead of Report an Issue--which is intended for reporting specific problems (bugs) with the different filemaker products.
I took a quick look at the Filemaker 11 Lending Library Starter solution. (I assume that's what you mean by "lending library".) You'd need to modify the design and function of the database (That's why they're called starter solutions.) in order to gain that functionality.
How familiar are you with how to develop databases in FileMaker Pro?
Not fluent in database development, but I also think there should be that functionality built-in already without any major modifications. It isn't a esoteric demand, seems like something others would need to do as well. Let me know if you have any detailed instruction in how one would go about doing multiple asset check-outs without re-selecting contact.