Membership and its application for our purpose(relating to Database usage)

We have batch create and are looking into both membership plug in and blog create…

This is a multi-site..

I want to accomplish the following.We have 2 themes that need to work together at the data base level..

One theme is a video tube site which houses 1000+ videos with imbed codes, from youtube ,vimeo etc..Corresponding thumbnails,tags and descriptions..

A second theme we built needs to simultaneously be able to access video/data from the tubesite (when liked)into user/members individual cms…

Our tube site allows our users to create a liked page .We want that info accessable to the 2nd site..

question one is does membership allow for a way to have our users ;logged into 2 themes at once . Or lets explore an alternative solution.. Obviously there are many way to solve getting data from one site to the other.. Just want to understand the possible tools membership may add to that mix.