hi,
well, it is possible but problems:
- probably you should use timer 2 with auto-reload mode for baud-rate accuracy;
- it loads MCU. For example even at 9600 baud you will need step to ISR each ~100µs. There it should saves used registers, checks for UART state/counter, sends current bit if it is need, prepares next bit or stop condition and restores registers then exits. Depend on code it eats 10...30% (or more) of machine time and so it is not full correct debugging as Erik already mentioned: your program execution will be terminated often and often; moreover it uses additional program modules which may influence on main ones etc;
- if you need not only to send but receive a data aswell then think about start-bit detection. Either use INT0/1/2 pin or polling method. First one requires defined pin; at second case you must increase timer speed for correct detection (at least for time while you wait for start-bit). It is because you should use 2-from-3 scheme to avoid false bits detection.

BTW, why just not use one of current UARTs for debug purpose? For example, your MCU_A is connected to MCU_B via UART. Then take TX of MCU_A and connect it to RX of PC or what you use as debug machine. Then monitor what MCU_A sends over the net. You may even mask messages for MCU_B and PC with 9th bit mode - it requires only minimal program modifications (indeed, if you have not already use this mode).

regards