Dave Staton was good to deal with. I asked several questions; he pointed me to the right documentation, & answered them all. So, I ordered the kits.
The two installs I did (mine & the wife's) are both done with one stabilizer; you can buy & install a second one for about $10-15 I believe. You would just need to drill an additional, matching hole on the right side of the motor mount channel. The motor mount channel is a nice solid piece of aluminum, & I haven't seen the need for a second stabilizer.
One of the real handy things I found out is that the rubber electrical tape, (the kind that bonds only to itself,) is GREAT for this sort of thing. (I'm not talking about regular, vinyl electrical tape. I'm talking about the rubber stuff, where you peel off a liner. It only sticks to itself, and it fuses to form a single layer of rubber. It worked great around the top of the fork, where the mounting bracket clamps on. It also is a very quick means to secure cables to the frame.
The double seat is great. Your weight is more distributed, you don't risk nerve damage affecting areas you really don't want to risk, and as each half swivels independently, it is very comfortable when peddling.
As far as the balance - it is a little heaver to the left, and when riding, the bike is probably a degree or two to the right.
I have only two issues with the Staton kit. I got the standard throttle, and you're squeezing a short, two finger trigger grip; after 10-15 minutes, it can cause tiredness in the throttle fingers. I'm probably going to extend it. (or replace it with a twist throttle)
The second issue is that the Mitsubishi motor has an built-in kill switch. I've got to pop the cover off to see what I have to do to add a remote kill switch on the handlebars.
Other than these two issues, I've been real happy with the kit, and the motor. It's got good power and runs smoothly. It's a little louder than the wifes honda, but, not obnoxiously. And, when we run at the same speed, I'm backed off a bit, so it's not much louder at all.
Once I get the original tires worn down & changed to ones with a smoother tread, it should be REALLY smooth.