You can try "multiple email signatures pro" from the app store..haven't tried it though. Or if jb try mailEnhancer 2 from cydia....which integrates into the native mail app allowing for multiple signatures....
I saw through a google/bing on topic a post from Lifehacker Australia that explains how to work around this problem by using shortcuts. General Settings, Keyboard, shortcuts. The drawback is that it is a phrase and you can't do multiple lines unless it is wraparound. The cool thing is, if you have an addy and phone # you can shortcut 123 and the addy and phone appear. Hit return. Unfortunately it's a workaround, but a great one compared to the alternatives.