Unfortunately the DS18B20's use high-speed signalling which is too fast for a PICAXE to bit-bang from a user's program. 1-wire commands are bit-banged from within the firmware rather than using an on-chip peripheral so the only way to access 1-wire devices is through the 1-wire commands provided. That means multiple DS18B20 on a single pin for the M2's is not possible.
The only practical way to do it would be to move to an X2 PICAXE or add an additional PICAXE which can read multiple DS18B20's and provide the results via a single pin.