I meant space the other way - it looks pretty deep, but really quite narrow.
(I am biased, mine has about 600mm free space between guide-rails)
I think it can only get away with the one handle because it is so narrow - which means that the centres are close together and the screws can also be bound together by the end brackets which helps them to move as one.
On mine, and I suspect the Veritas (but I haven't tried it), the chain/sprockets do the bulk of the work but because the screws are independent and the centres are more distant, nipping the vice up fully on both sides needs the last eighth of a turn or so done on both handles together.
Well, more accurately, it might not need it, but I always do, as I stop when it starts feeling wrong to put that much pressure through the chain/sprockets.
In practice, I've never had an issue with the fact the handles spin around , although I can see why you think that and remember thinking the same thing before I built it.