how to handle gem versioning when gem is in subdir of main project?

We have a git project that has a directory structure similar to...

/big_project/web_ui/rails_app_one /big_project/web_ui/rails_app_two

within the rails app we want to use our custom gem under

/big_project/web_ui/our_custom_gem

since this gem is shared sometimes rails_app_one may be using a silghtly different version from rails_app_two

if i try something like: gem 'our_custom_gem', '0.1', :path => '../our_custom_gem'

the version fails as not being found if the local version of the gem is lower than the current local checkout.

For example if rails_app_two need gen v '0.2' and rails_app_one needs gem v '0.1' rails_app_two will work fine, but rails_app_two fails w/ error saying it can't find the version

if we try to use it from it's git repo (bitbucket), it still seems to behave the same way (altho i'm not sure i'm using the right params in my gemfile)

In any case I have not found a way where I can do the following:

have app running under /big_project/web_ui/rails_app_one use one version of a gem that lives under /big_project/web_ui/our_custom_gem (in the same repo) and /big_project/web_ui/rails_app_two use a different version of the same gem

Any insight greatly appreciated