Had a quick look at the belt specs - not that much info really - mine aren't overprinted with a manufacturer.
As far as I can see on their site, tension should be 100N. Without getting involved, let's call it 10Kg or 22lb if you are metrically challenged. That should be easy to pull by hand without resorting to a screwdriver lever
Similar specs at gates show +\- 0.03mm at 1200mm belt length variation (I'm assuming manufacturing variation).
As for stretch, I don't know.
Perhaps someone with higher mechanical knowledge can suggest what a ballpark value might be for HTD 3M style (not the company) 9mm wide, 3mm pitch. I believe the core is glass with neoprene casing - maybe kevlar is better (although I can't think why kevlar would stretch less than glass).
As an aside, gates have a wee tension gadget where you pluck the belt and it listens for th tone - you put in some values and it tells you what tension is. I have an app (free I think) on my iphone that I use to tune my guitars, maybe that would work - given a similar overall length ?
Need to ask the guys at motionco methinks.
BTW, locking your two Y leadscrews together when both are being driven is not a great idea. However, driving one and belt to the other is ok - but you're then back at square one
Its really only a problem when one fails (note the word 'when'
I know of lots of systems that use two motors 'though - shopbot springs to mind.