You cannot cast twice the same endless spell, but nothing stops your opponent to cast it. So you can have 2 copies of the same endless spell on the battlefield, as you cast one of them and your opponent casts the other one.
Slightly off topic: does the caster manipulating them have to stay within 9" to benefit from Slow Down Time?
Yes, you don't get the benefit for casting them, you have to activate them. This doesn't need to be the same wizard that casted it
Right, but which ever wizard manipulates them must remain within 9" to continue to benefit from the additional spell cast each turn, correct? I've been using the Cogs frequently of late. I cast it with a Starpriest, manipulate it with a Slann and then often teleport the Slann away for a deep-strike. The Slann would have to return within 9" on my next turn to benefit from an additinal spell cast again in that turn.