Seems to me it would be much, much easier to add bridging between the joists or maybe an additional beam under them halfway between that span than it would be to sister or laminate them...
Heck, even screwing a perpendicular 2X4 to the bottoms of the joists would help them "share" the load and stiffen things up. You can't need much more with that sub-floor.
Good luck.

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