isn't it possible to connect them at gen3 ports?
also i doubt that there is any or its very minimal difference between gen2 and gen3 in speed.
its like theoretic speed, etc even connected to gen 3 ports the will not run at 6Gb/s speed,
but down to the SSD's speed itself, which i think gen2 bandwidth should enough not to slow down them
If possible to test, connect one of the SSD drive on gen2 speed and test his performance.
then connect it to gen3 port and retest, then compare their transfer rate.