Dave. i have several boats with only one ESC and two motors, and none of them start up both together at very low speeds. You will always get some variation in motor, friction of couplings, prop shaft and so on to affect initial start. Unless the motors are still not together at slow speed, rather than just VERY slow speed, this should not be any problem. I presume you have no tweaks available on the speed controllers? Also , is this in the air , or boat in the water. If in the air, I would not worry and plonk it in the oggy first to see what happens then.
If I try very hard, I can get the 4 air props of my Sunderland to start one at a time, always impresses!
In any case, would you want to go that slow on such a magnificent fast boat ??
Ashley