'Mechs and most other units cannot shoot at targets within the same hex per TW p. 106, even in this special case. So there's no issue here.
Thanks!
Maybe it would be worth adding a sentence to the same effect in the BattleMech Manual?
Also, combat between units in the same hex is allowed within building hexes, which is something my first printing of TW fails to mention on page 106 (apparently unaddressed by any errata, but I hesitate to submit a report without having a more recent printing to check). In that case, page 175 of TW provides the Special 'Mech Hit Location Table for when 'Mechs on different levels within a building fire on each other, but this table and the reference to it is omitted from the same section in the BMM on page 72 (no complaints here, I'm all for simplicity). But the third bullet under Combat Within Buildings doesn't provide enough information to resolve this situation:
Always use the Front/Rear column of the standard Hit Location Table to resolve hits (see. p33). If the target is in the same hex as the attacker, treat it as if it were in the attacker's forward arc.
Should players always use the 'front' of the Front/Rear column? And if not, in what situations should they use the 'rear'? The Total Warfare rules also work quite a bit differently for combat within buildings in general, since they only default to Front/Rear for attacks against prone 'Mechs originating from different levels.
There's a bit of an issue within the Total Warfare rules on page 175, too, because they also don't specify which armor facing an attack should resolve against for attacks within the same hex (except for vehicles, in which case the side hit is randomly determined).
Apologies if this should be a new post!