Structurally speaking it makes no difference where the added joists are placed assuming the floor sheathing is adequate to span 16".

But pay careful attention to how the new joists are supported and braced. Added joists are often sistered because it is easier to support and brace them. It's a carpenter's option.

The new decking will have only a slight effect on the stiffness of the floor. It is the bottom of the joists that must be reinforced in order to reduce deflection.

I still recommend cutting LVL's to fit at the 15 ft span. If you are going to his much trouble you should spend a little more for stronger members in that area to reduce bounce.

Macv - thanks for helping. Can you explain what you mean by reinforcing the bottom of the joists? I don't follow that part.

We are looking at LVLs and sistering 2x6. The problem is the LVLs are only available (here) in 8 or 12 inches. I don't have the headroom upstairs to add the additional height. We are doing a stop-gap until a full remodel next summer.

THanks
Jennifer

You can special order the lvl's equivalent of a 2x6 from most any reputable lumbie that deals with engineered lumber,(most do these days). Problem is that the lvl's will likely run a little taller than a dimensional 2x6 which is nominally 5-1/2". I believe the lvl's will run 5-7/8". Any competent carpenter can scribe them down to the size of your existing 2x6's in fact could do the same with 8" lvl's which come out at a 7-1/2" height.

Who will be doing the work?

Framer/carpenter will be doing the work. My neighbor flips houses and the guys on her crew will be doing it. (professional guys, not a home-handyman)