Warming to irrigation control in CTC

Need some help guys,

I have an installation that has two water supplies - one is a boosted pressure (which will/can run multiple irrigation zones) and the other is a very low pressure mains connection (that can only run one zone and maybe a tap/shower if no one else in te street is using anything).

Would love to be able to use a pressure sensor into a gen input and depending on the available pressure overlap or run multiple zones at the one time unless the pressure goes back to mains and then run only one zone.

Any suggestions? I am hoping I can continue to use the irrigation control as it doesn't ever miss a schedule.

Is there any access to the overlap function through logic?

Any ideas would be greatly appreciated.

Cheers