two primary registers in one meter for a secondary meter device

There is this scenario is our context ... which results in having two primary registers against one secondary register, its like

one primary meter ... which is a TOU (peak off-peak) meter ... has a child meter which is not a TOU meter. Now, the consumption of the child meter is to be subtracted from active peak and active off-peak registers in the primary meter proportionally.

Do we need to write a functional module and define a new register relationship for this?