You can't access the MAX internal RTC for external timing or clocking. The date-time is derived from a large counter, not like in the Dallas part.
When the MAX is on and has a fix, you can use the timestamp in NMEA messages to set the MCU clock; the MCU should be able to coast through the MAX off periods with reasonable accuracy.
Backup power is used to maintain SRAM that contains all kinds of valuable data required for hot and warm starts, to retain configuration settings and calibration data.
[Edit] There are some u-blox parts that allow an external RTC oscillator which could be shared with other devices. The date-time for the other devices would have to be generated as described above.