How can I "give" a forked process a diferent dynamic library than the one it "asks" for?

I want to know how I can "give" a forked process a different dynamic library than the one it "asks" for. For example, if the fork "asks" for the library "foo.so" the main process "intercepts" that and instead provides "bar.so". Considering I don't know much about how dynamic libraries are loaded, it's very much possible that this is impossible.