Any of them can be master on wordclock. This means that they will stay in synch to each other.
Even easier is to use the c.link connection (all you need is a cable), this transfers clock as well as start and stop commands so the boxes starts at the same time as well as stays in synch.
The "t" is for timecode, something used only in film production. The idea is that each file is accurately timestamped. This makes sense if you have several cameras and maybe several sound recordings going on. The sound and video then "magically" gets placed in relation to each other in the editor software and you save time in that phase of production. Not much use for the general taper though, a normal 702 will do the job and cost less. And yes, it is not a bad idea to go for 2 x 702 instead of one 744t.
Gunnar