Flash rust occurs when the tank is wet inside - that happens during a hydro test, or during O2 cleaning if the tank is not quickly dried or if a rust inhibitor is not used.
Condensation can also occur in a tank if it drained and then de-valved while still cold. So after you drain it, let it warm back to room temperature, then de-valve it.
Flash rust could also occur if you leave the valve off for an extended period of time and allow humidity in the air to enter the tank over a period of hours (if it is really humid) or days (under normal conditions.)
But in short just removing and replacing a valve will not cause flash rust if done properly.
That said, most people have dedicated singles and doubles as setting up doubles does take some tinme and does require the tanks to be drained. In some cases, diver will use single tanks as independent doubles using a set of travel bands to quickly band 2 single tanks.
I that case the diver will switch regs during the dive to keep the gas balanced between tanks. In a situaion where the diever wants to maintain a 1/3rd reserve (deco diving, etc) the diver woudl breathe 1/3rd off the first tanks, switch to the second and breathe 2/3rds of it (with the mid point of the dive occurring after the first of the two thirds is used), then switch back to the first tank to breathe the second third from it. This ensures that either tank has enough gas to get you back to the start point in the event of a gas failure.
Some divers make it a lot more difficult by switching every 300-500 psi, but they gain no real advantage. They ensure that the entire loss of gas in either tank would be equal to the other tank, but 50% of the time they will lose more and 50% of the time they will lose less gas than they would with the above method and create more task loading to no advantage in redundancy.
