In series, the voltage of two cells adds, and the same current runs through both batteries. Thus, the driver sees 6 volts (before the batteries sag).
Off the top of my head, I believe you are correct about emitter current. Roughly speaking, an XM-L2 or XP-L needs about 3 amps in order to produce 1000+ lumens. You can check that on the Cree data sheets.
Emitter current, however, is not the same as battery current. On one side, a driver has an input voltage and current. On the other side, it has an output voltage and current.
Now we get into territory where I am not knowledgeable. I believe the forward voltage of, say, an XP-L, is around 3.3 volts. That is the output voltage. We estimated output current above, at about 3 amps. We know that 2xCR123A (before sag) gives 6 volts. That is the input voltage. What we don't know is the input current.
Without trying to put a number on it, let me suggest that a typical driver won't need 3 amps from the batteries in this circumstance. My guess, however, is that it will need more than 1.5A! Probably, we are in the neighborhood of 2-2.5 amps, but that is only my wild, seat-of-the-pants guess.
Instead of guessing, of course, what we need to do is to measure the current. This is especially important because we have seen evidence in highly driven flashlights that CR123A batteries often exceed the maximum continuous discharge currents they are designed for.
By the way, CR123A can be used safely in all of the flashlights you mention. The trick is keep them on medium and low. It is only the highest modes that pull too much current.
