Is there a reason why you want to use MAX232? There are other device types that would handle this job much better and support multiple drops. If cable length is an issue, then almost any of the very low-cost RS485 driver/receiver pairs would do the job and they don't demand the bipolar supplies. If you do need RS232, then MAXIM certainly has charge-pumped devices with more drivers and receivers. If you do have the means to control which device is driving, either a multiplexer, e.g 74HC153 or a tristate buffer, e.g. 74HC125/126 would do as well. With a little imagination, a GAL16V8 can provide a multiplexed path between drivers/receivers and the MCU's, but someone's always got to be in charge.

Perhaps a little more information about the precise requirements and timing would be helpful.

RE