The trigger or vertical output from one oscilloscope can feed the trigger input of the other oscilloscope however it is usually better to just trigger the oscilloscopes off of the same source using their external trigger inputs.
Unfortunately unless you are crosseyed you won't be able to look at two screens at once.
A TDS724 is big enough that one can be stacked on the other. I have actually done this with 2230/2232 oscilloscopes stacked on top of each other and it is actually pretty handy. And as a bonus, the oscilloscopes can still be used independently to make measurements that a single timebase oscilloscope cannot. This duplicates the functionality of some impressive old instruments like the Tektronix 547, 555, 556, and 7000 series which could essentially operate as two independent oscilloscopes with one display.