I bought a pair of cheap 6x9's that I want to use for rear fill. They will
replace the factory rears that are currently in there. I want to run them in
parallel with my kenwood splits that is in the front.

Problem is that the 6x9's have a sensitivity of 92 while kenwoods have
sensitivity of 91. They will obviously play louder and mess up the
soundstage.

The rears currently run off the HU but I want to play the 6x9's off the amp.
They have a nominal of 60W.

Is there any way I can control the rear 6x9'x while they are in parallel
with the splits?

My amp supplies 100W rms per channel.

I bought a pair of cheap 6x9's that I want to use for rear fill. They will
replace the factory rears that are currently in there. I want to run them
in
parallel with my kenwood splits that is in the front.

Problem is that the 6x9's have a sensitivity of 92 while kenwoods have
sensitivity of 91. They will obviously play louder and mess up the
soundstage.

I doubt they'd be louder. The mere proximity of the front speakers will add
several dBs when compared to the rears (not to mention the shape of the
ears!). Also, you don't know what the sensitivity looks like across the
entire spectrum. Sensitivity measurements tend to be based on 1kHz tones.
The rears may indeed be too loud for your liking, but I don't think they'd
sound louder.

The rears currently run off the HU but I want to play the 6x9's off the
amp.
They have a nominal of 60W.

Is there any way I can control the rear 6x9'x while they are in parallel
with the splits?

My amp supplies 100W rms per channel.

There's no easy way. Personally, I probably wouldn't do it. You'll be
running your amp hotter, you'll have this problem, and you'll be slightly
robbing your fronts of power (probably not much though).