Is it possible to do direct serial communication between two 8052 MCUs (TxD connected with RxD and vice versa), operating on 3.3V without a MAX232 converter?
When you control both ends, you can communicate any way you do, the only point is achievable speed and error rate.

If both uCs are on the same PCB, I see no reason not to connect them directly w/o transcievers.

If both uCs are in the same box, and you connect them directly w/o transcievers you should be able to achieve a rate in the 9600 range without any problems.

If they are not in the same box I would not even think of not using a transciever, be it 232, 422 or 485 that the app requires.

I think the CMOS 3.3V levels are not sufficient for RS232 communication and are not conform the RS232 standard voltages.
Nobody say you need to have RS232 levels if you "own: both ends, as long as both ends are set up the same. RS 232 is required, if you communicate with e.g. a PC which expect them.

Erik