如何从命令行调用旧版本的gem？

Say I have two versions of a gem installed (somegem versions 0.10.6 and 0.10.5) and I want to run the earlier version from the commandline. Do I have to uninstall the newer version? Is there a way I can use a flag to specify which version I want to use? Something like... Say I have two versions of a gem installed (som