I am running a Multiplus 24/2000/30 for a few years now. I would like to add a backup unit ( Smart 24/2000 ), just in case the Multiplus should fail. Would it be ok to wire the 230V outputs together, and have a battery selector switch on the 24V+ supply, i.e. only one unit can operate at any one time. Is either unit ok to be connected to 230V without having a battery connected ?