(09 Aug 2015, 3:10 pm)S813 FVK wrote Doesn't look like they do interwork unless (and quite understandably), there is a long layover at each end.
For example, an X10 leaves Newcastle at 11:00 and will get to Middlesbrough at 12:20. The next X10 out of Middlesbrough is at 12:30 meaning that it would be the next bus out and would get back to Newcastle at 13:50 . The cycle would keep going and works in the same way for the X9.
As i say, with it being such a demanding route for drivers, they may be given 40 minutes layover at each end allowing buses to alternate between the two routes.
As far as I'm aware the X9/X10 don't interwork so it's three buses on each service. So like in your example it leaves Newcastle at 1100 on the X10 then arrives back as the X10 at 1350 then back out at 1400 on the X10 to Middlesbrough.