Now the question is can I use a double pole double throw relay of needed power rating as the change element between the feeds or should there be longer off time when changing from feed to another? The idea is to keep it simple and thus avoid timers and/or PLCs
If you have a fast acting control over the inverter (a way bringing it high impedance aka HiZ, so all transistors off, with delay less than a ms), it may be enough to provide sequencing: Inverter powering - Inverter HiZ - relay switchover (with a direct inverter electronic controlcould be at the same time, the relay being slower takes care of the sequencing) - generator powering.
To swirch back, the sequence is exactly opposite, but you have to provide delay between the relay and inverter activation (or wire it via interlock contacts).
Otherwise if you do not have such fast inverter control, you need two separate relays and/or contactors, with a safety delay gap longer than the relay turn off delay (or using contactor interlock contacts to guarantee the otger is activated only after the first is mechanically really off).
Just a simple DPDT may flash over, because there is not enough separation when the eventual arc is burning over contacts, dont forget there is no guarantee about the switching sequence at all, so one contact may just switch over and cause the flashover while the second has not yet moved at all.
