I'm turning my 100 year old attic into a living space. I'm concerned about the floor/ceiling joist spans. The joists are 2x6

and the span in question is 16 feet. Although it is covered with 3/4" tounge and groove and has lasted this long, i'm not sure it will hold up to regular traffic. Can I simply add another 2x6 along side(glued and nailed) the existing joist? I've done this already on the other half of the attic, which is a 12 foot span, and it seems to be quite sturdy. I'm planning to use the space for bedrooms.