By Odin.Equivocator 2009-09-10 21:00:18
You can only have 1 GP contract at a time.
You can get all the good items for fishing, but not at the same times as getting LeatherCraft items.
After you get your LeatherCraft items you can start the new contract with the Fishing guild, at this point you will lose any Leather GP you have left over, but not the items you have already bought (including Key items).