I am creating a social network website with Buddyboss as the central theme. I also want to have the freelancing features that Workreap offers. Other plugins like Gamipress also provide some additional functions which must be shared for both themes.

I want people to register only once on the main site, and if they go to the freelancing segment/page, although within a different theme, the experience remains seamless.
In other words, they don’t need to register again in Workreap; Also their profile name/picture, points earned in Gamipress, membership level, etc. in the Buddyboss theme is retained (as much as possible).
In fact, I don’t want to treat the Workreap site as a completely separate and disconnected site.

Is it possible to install the Workreap theme on a subdomain? Should I use the same database for both the main domain and subdomain?
Or should I do something else altogether?

Any answer to any part is appreciated.
Thanks a lot!