So sistered floor joists need to touch the wall?
It’s complicated and it depends. You’re using 2x6’s so there can’t be extra large loads, so you’re probably ok. 

The reason the joists need the correct amount of bearing area is so that the joists don’t crush where they bear. Extremely large loads tend to crush the fibers of the joists at bearing points, unless they have an extra amount of bearing area. You are using 2x6’s which don’t support much so I’d guess you have plenty of bearing with one joist. 

A bigger problem is deflection (bending).  If you over load the attic, the joists will bend and crack your plaster. I’d keep the heaviest loads against the walls and the lightest loads in the center of each span.