I have a spare bottle of Marvel Air Tool Oil.
Is it OK to mix it with regular MMO and use it in
the gasoline.  Supposedly they’re both identical
formulations, except for a small amount of
anti-oxidant in the Air Tool Oil. 

What proportion would you suggest to mix the two
to dilute the anti-oxidant.  Don’t want to add too much
of what may be a fire retardant to the gasoline.

First, you are correct that both the Marvel Mystery Oil and the Marvel Air Tool Oil are similar formulations, but not exactly the same.  The Marvel Mystery Oil is an EPA registered fuel additive while the Marvel Air Tool Oil is not.  For this reason we can not recommend using a blend of the two in your fuel tank.  However, is you were using it something like a lawn tractor or other off-road gasoline powered vehicle (EPA registration does not apply to these vehicles), you can safely blend them in a 50/50 ration.