Obviously giving access to more than 1 site, and mentioning that you do so in the tour, will help conversions.

If you also want to help retention, you'll be far better off if each site has its own members area. Then you can also do rolling updates in various site's members areas, which will help with retention even more.

We use MAS from Mansion to manage all of the member's areas from 1 interface, so from our perspective it's as if we're managing 1 site, but from the consumer's perspective, each members area is independent from the others.

It's $4,000 but worth every penny.