Heating Help

split a 1 zone monoflo into 2 seperate units

I am looking to split the current 1 zone system into 2 zones or 2 separate boilers.

I have a 2 family house with a 1 zone monoflo system built in 1955

There are 3 rads in the basement and a 1st floor 3bdrm apartment and a 2nd floor 3 bdrm apartment.

This way the tenant can keep the heat at the temp they want and pay with a separate gas line.

How is this done?

If we split the 1 system into 2 zones there is no way to separate gas use correct?