It depends on the systems and the PLC of course. Basically you want to run two separate programs based on the time. I would be inclined to have an external event (on a digital input port) from a timer or 7 day clock and also from a test switch. This would make development and testing easier. Then you could have either separate programs if that is allowed or separate statements that are executed or not by the state of that input pin. This depends to some extent on how complex the two systems are, and how much they have in common.
It might be possible to have the same sort of thing based on a timer internally. The best way to find out is read up on what is there, and try a simple program to see if it works. It could be an "if... then..." structure, or a case or switch structure, based on the external event or the timer. Note that some PLCs are programmable in basic or C other than ladder logic, which might turn out to be easier (it seems to me anyway, being more familiar with these). I expect that the internal timer running for 7 days might be an issue, or lack of program space or memory if the program is too big. Another problem could be the number of i/o ports if there are separate sensors and controls for each system, as these probably have to be kept separate. It might be possible to switch the i/o over using relays though, reusing the same inputs with different devices. Also remember the timer or external event can be used to set a variable that can be used globally in the program.