All may attack.
In constrast to modern rules, the rules for wargames back in the 70s left a lot to the interpretation of the players. This is also why the rules are so much shorter.
Paragraph E of the CAT rules states...
At least one engineer unit must be stacked with at least one infantry or cavalry unit, on at least one of the hexes of Assaulting units.
It's subtle, but that last clause includes the plural "hexes" which means units in multiple hexes can attack the same target hex.