Yes. But some care is needed: Theoretically you could connect all the TX pins directly to the input of MAX232, if, and if, only one is sending data. The according micro must be 'enabled' to send then, by the master, while all others are put into 'idle'. This assumes, that derivatives with quasi-bidirectional port configuration are used, where the inactive TX line is weakly pulled-high by a weak internal pull-up.
But there's a certain danger, that the micros run ill and more than only one is driving its TX line. So, you should connect all TX lines of the micros to inputs of an AND gate, with its output connected to input of MAX232. Means, if you have 4 micros, then you need an AND gate providing 4 inputs.

Kai