Parallel installations of 1.8 and gems

I’m trying to get both ruby1.8.5 and ruby1.8.6 installed on the same
machine, but I’m running into problems now finding gems.

I’ve got the libs for 1.8.5 in
/usr/local/ruby/site_ruby/1.8.5

And for 1.8.6 in
/usr/local/ruby/site_ruby/1.8.6

I downloaded the gems tgz, unzipped it and ran setup.rb with ruby1.8.5

It seems to have installed gems in /usr/local/site_ruby/1.8 so when I
try to require a gem it fails.

Is there a way to fix this? Is there a reasonable way to share the
gems between 1.8.5 and 1.8.6?