Same user database

Hi there,
I am about to set up my second V forum and wondering if i sould go for a separate installation of VB or not.
My requirements are the following:
-2 websites: www.domain1.com & www.domain2.com
- www.domain1.com is already live and running
- www.domain1.com forum URL is forum.domain1.com (sub-domain)
- the 2 domains are on the same server
--> i'd like www.domain2.com forum link to be forum.domain2.com
--> i'd like the 2 forums to share the same user database

I know that it's easier if the 2 forums share the same install but how would i then
--> get domain2.com forum url to be forum.domain2.com

I'd prefer to have two install and then share DB but not sure how easy this is for a non coder...

Thanks in advance for any hint.