In UVMapper Classic (the free version), select all of one shoe. Then do Edit > Assign > to Group and give it the name lFoot or rFoot as appropriate, making sure that the spelling is exact. UVMapper will ask if you want to create that group; say yes, and save the model. If you built one shoe with the intention of mirroring it, you may want to do the grouping before combining the two meshes. That way, both shoes will have the same mapping and your texturing will be easier. :)