You can't use pins 27 and 28 they are reserved for the eprom used to configure a hat. This can be verified by the note included in the schematic for the 40 pin header.
The HAT design guide includes this:
On a Model B+, GPIO0 (ID_SD) and GPIO1 (ID_SC) will be switched to
ALT0 (I2C-0) mode and probed for an EEPROM. These pins will revert to
inputs once the probe sequence has completed.
The only allowed connections to the ID_ pins are an ID EEPROM plus
3.9K pull up resistors. Do not connect anything else to these pins!
While the above specifically mentions the model B+ this is true for all Pi's with a 40 pin header.
However, the I2c bus supports multiple devices as long as they have differrent addresses (this is likely the case with the two different devices you have - though most devices also support changing the address). So your problem really comes down to getting access to the I2C pins which are inaccessible due to your Z-Wave module. A board like this one
can allow you to access the I2C pins which are inaccessible.
You can also remove the Z-Wave and connect it and the RTC to the Pi using a breadboard and jumper wires.