MOVs will not help your crash. MOVs are useful only on power surges. For projects like OpenSprinkler, they help mainly during electric storms. Lightning strikes on the earth can send large currents through the ground for long distances. The large ground currents will induce voltage spikes in wires buried in the ground. Since the sprinkler valve wires are most often buried, this brings the voltage spikes right back into OpenSprinkler. The MOVs help dissipate these voltage spikes before they damage OpenSprinkler.
I suspect that your problem is that your 24V power supply is undersized for the current draw of your sprinkler valves. With only one valve on, the 24V power supply is OK. With two valves on the 24V power supply is marginal. As the 24V transformer heats up, its output voltage drops and when it gets too low, OpenSprinkler crashes. I would monitor the +5V on the OpenSprinkler and then activate two valves watch the voltage value and then touch the transformer to see if it is getting hot. My guess is that the transformer will start to get hot and the +5V will start to droop. If you find that the above it true, then get the current rating of a single sprinkler valve. You have to check with your manufacturer, as they vary widely. Then your 24V transformer should be rated at 2.5 times the rating of a single valve. The extra 0.5 is for current used by OpenSprinkler and saftey margin. If the 24V transformer appears to be sized OK, then something in your system is out of spec. If you have a AC ammeter setting on your multimeter, then you can check if a valve with too high a current draw or whether the 24V transformer is not putting out its rated value.