Typically if one can't prevent a system from connecting two voltage sources of the same nominal voltage value together, one effective protection method is to have each voltage source wire through a series diode (a low Vf diode is best) and then the module well draw current from the common cathode wired together (assuming a positive volt is being used) such that neither one can force current through the other and the device will just draw current from whichever voltage source is slightly higher then the other. There are more complex methods of isolating (like the Uno uses) but simple diode ORing is effective.