sharing one 110VAC circuit with two high amperage devices
You didn't give much detail, so this is only a high level knee jerk reaction.

Get a relay appropriate for switching the two AC feeds.  Either make sure this is a break before make relay, or you can use two separate relays that you manage yourself.  The unenergized state routes the power to outlet A, with the energized state routing it to B.

That was the easy part.  The hard part is detecting which state the relay should be in.  One way or another you'll have to measure current on B to decide when to switch it off.  This is probably best done with a current transformer.  This can also be used to detect current draw on B when it is switched off.  Instead of switching off B completely, there is a resistor providing a little bleed current, like a few mA.  The relay for B shorts accross this resistor to turn on B.

When device on B tries to draw current, it will get just enough to detect via the current transformer.  The current transformer output is conditioned and presented to the A/D input of a micro, which then looks at the magnitude of current and decides what to do.  It can also do a few useful things you didn't mention, like wait a second or two to switch either direction so as to not respond to short glitches.  It can also easily manage two relays to perform break before make.