Basically the KV rating has no big effect other than having more Top Speed. You will just have more Amp flow at the same speed, putting everything under more strain (Contoller, Battery, Motor). Losses will be greater, due to heat & increased resistance in cables. Our motors have an efficiency of 90%, measured at 12S. Most simple BLDC motors are around 75-77%. Don't believe the 95% efficient everyone claims! Cheep motors use cheap magnets & low quality copper resulting in less efficiency, especially at high amp flow & low voltage. The reason why they cost 80 - 90 $ is because they are cheap quality! The VESC 4 is at its limits when using our Motors at 50Amp Current draw. VESC 4's work fine, but they are the bottleneck in the power not the motor.
Using a higher KV rated motor will have the effect of loosing torque at Amps your system can supply and being less efficient at cruise speed. Sure your motors are bigger and might compensate that a bit. KV seems to be a religion, but if you ask Benjamin, (Benjamin Vedder is the Dark Lord of E-Power!!) he would say: Copper fill is the only thing that really matters if your KV is somehow within the range of efficiency. Good motors have a better copper fill (hand wound), cheap motors have less copper fill & are machine wound. KV should always correspond to the realistic average speed. Peak performance at realistic average RPM! At 40Km/h we are, using our 136KV@1Nm setup, close to peak performance of the motor. Already to fast, as you can see. You never go that fast in average! This is why we use the 118KV@1Nm Motor.
All I want to say about sing the 190kv motor with 9 inch wheels is expect your setup to be not very efficient. 9" Wheels would make your motor spin even slower at the same speed, demanding for even lower KV to become efficient. Believe me, our motor's a very well balanced for the application, but of course you can always go crazy. Biggest Problem is the wheel slip anyway: When your wheel slips and get back to grip, the Ampflow shoots up very very fast. You can correct the setting in the VESC, using FOC (T: 500,0 µS).
A Twin 136KV TRAMPA Setup is a beast! It puts the tires under massive strain already! Bigger Motors just create more wheel slip & will of course wear out tires faster. We also have sensors attached to the motor stator. This feature is imperative, when riding MTBs. VESC 6 will make full use of this feature but even VESC 4.XX likes the sensors a lot. Temperature sensing should also be done for safety reasons. Another issue you could get: Your motors could output more power than ours, putting everything under more strain. We tested our drives with our motors. Dropping in much stronger motors is an experiment you are responsible for! I'm not pessimistic, just want to clarify things.
I would not connect both drives to another! Our drives are 100% aligned to each axle shafts. Due top tolerance in production of the Spring Truck Hanger MTBs never have two perfectly aligned (in line) axle shafts. In consequence, the drives will never be 100% parallel. If you align the drives to another, your drives will not align with the wheels. Slim chance you can get that straight. But for sure, belts swallow some tolerance. If one drive would become slightly loose, due to vibrations and abuse (MTB suffer alot), you would put the other drive under strain and would not realize something is not quite right. And anyway, technically speaking there is no good reason to connect them. We made our Drives, so you can use them as singe drives without any problems. Two singles are a TWIN, but each drive is set free from the other, so that they don't affect each other. So you gain nothing by joining them together!
Hope that helps you, Frank