ABYC requires that it is not possible to engage both shore power inlets at the same time.
Send an email to boatpoker@gmail.com and I will reply with the ABYC electrical standards in pdf's
Mutual exclusion can be accomplished via interlocked breakers as has been shown here, or using a selector switch with break-before-make connections (which is typical).
That should have been a private message. I posted by mistake. Please ignore.