I like both parallel twin and vee-twin.
Wouldn't matter much to me.
I think parallel would be easier for them to do.
In the old days, the singles and the twins both had basically the same frame.
I think they could do that today too.
As for the high speed stability, I don't see any problem there.
The old Bullet can be nice and stable at 100mph(with a modified engine, of course).
I see absolutely no reason why they couldn't do this pretty easily, and in an affordable way.
If they were to do this parallel twin, I would recommend that they style it like the Interceptor Series 2. It would be another type of "retro bike" that could fit the heritage, and I would rather see Interceptor styling than the older 50s Meteor styling.
Broaden the appeal, so there is a 50s look with the C5 single, and a late 60s look with the twin. IMHO.
But if they wanted to do it the least expensive way possible, they could just use almost all of the same frame and bodywork as the C5. That would make the fewest number of extra parts to build, and allow them to keep the price as low as possible, because of using existing production parts as much as possible.