You might get a better precision with get_tick_count instead of get_day_seconds, if you need it.
I was aware of this, but the precision is not too important (and the clocks are synch'd by hand) and I am taking high res photos over a 6 hour period - in my case I'm taking one photo every 30 seconds.
I assume cameras are on different balloons, or you could sync them perfectly via the USB remote and an external clock pulse.
They are on the same balloon, one pointing down, one to the side. This is purely a weight and simplicity thing. Using two ixus 70's I've managed to get the entire payload weight (everything except balloon) down to 745 grams. This includes the two cameras (286g), 4AA lithium batteries (94g) and GPS/Trasmitter/Antenna (124g). The radar reflector is the heaviest individual item at 189grams (I'm looking at changing this to a mylar covered carbon fibre frame).
I'll post more info in a more general location when assembled and ready to fly. This week I'm putting a 3 day old ixus70 in my freezer at -20 degrees c to make sure it still fires and to measure the battery life (If it's too low I'll need to have the batteries insulated and run wires into the camera). Lets hope it works and the temperature doesn't cause the LCD to crack!
Thanks for your comments.