As I understand it, the gate out is a gate generated by the internal note track - so if you have a sequence running on the pro2, your external gate out 'clock' will only trigger on the steps you have 'on'. This means you're not really sending a clock, but a series of gates - So your external sequencer will be either identical in rhythm to your internal sequence or completely out (unless you work out the timing). Basically, two parallel, in sync, sequences will be a very hard thing to do.
If DSI can make the CV outs fire triggers or vary the pulse wish of the CV outs (seems theoretically possible to me as they can produce 0-10 Volts) then you could have a clock generated from one of the internal sequencers routed to a CV out.
Currently, you can make a makeshift clock using CV steps alternating with high & low voltage, but this only clocks at half speed of the internal sequencer. Again, if DSI could incorporate different divisions of the internal clock speed into the Pro-2, this problem could be gotten around easily. Unfortunately, none of these options are available yet.