First time poster; I'm mostly a lurker 
I have the powered rack version of the Kemper and recently purchased a Mission Gemini 2-P passive 2x12 cabinet. I didn't realize that the cab is stereo with separate inputs for the right and left speakers. The kemper only has one powered speaker out.
Do I have any options? I tried a Y-cable with no success. Any knowledge and opinions are very appreciated.
I guess your only option is to go from the powered output into one of the Gemini's inputs.
To make use of the Gemini's stereo possibilities you'd have to feed the Profiler's monitor output into another external poweramp and go from there.
Quite cumbersome.
And welcome. 
If you don't want to run a stereo rig, I'd just recable the cabinet in order to work as a mono cab.
Y cable should work fine, is yours wired correctly (and test it with a multimeter)
We don't know which Ohm those speakers have and if you soldered your Y cable for parallel or serial configuration.
If your speakers have 4 Ohm don't configure those in parallel, because you would end up with 2 Ohm and might damage the internal power amp! This applies to both an Y cable as well as any cabinet merger.
The site says they are 8Ohm per channel. So a Y in parallel or series would be ok (4ohm vs 16ohm - Kemper manual says power amp works to 4Ohm).
Thinner and brighter than without the Y?
If so, that smells like either a) the drivers in your cab are wired out of polarity (has anyone been inside it to potentially rewire it incorrectly?) or b) there is some very strange wiring inside that cab - in which case talk to the manufacturer about how best to get it working in mono.
If it sounds thin and bright even without the Y then obviously there's nothing wrong with the cable and it could be the monitor EQ on the KPA that is at fault.
If each side works well standalone, but it sounds thin if both are combined, this might be a polarity issue inside your cab. Unfortunately there are no pictures of the connection panel. It says, there are speakon/TS combo jacks!? Perhaps there are sockets for bi-wiring and you have plugged into the wrong socket?
I'm guessing you've checked both sides of the Y cable for correct polarity?
If so, it does sound like something is wrong inside the cab (probably a driver wired with wrong polarity).
To kemper: think a stereo powered version is hopefully coming soon
