I don't know of any site on the web about multi-loop control. There are several books available at www.isa.org, however.
The configuration of a control loop where the output to a valve is the lessor (or greater) of two or more PID controller outputs is very common. It is known as override control. I have implemented the scheme a number of times with analog pneumatic and electronic controls and more recently using DCSs.
The outputs are combined using a device (or software module) called a low selector. The output of the low selector then goes to the analog output and to the valve.
There is one primary consideration: reset windup. At any given time only one of the control loops is manipulating the valve. If the other control loops have reset (integral control mode) the output of the pid algorithm will continue to move (called winding up). The solution is to connect the output of the selector (signal going to the valve) to the external feedback (sometimes known as reset feedback) of each of the controllers.
I hope this helps. If you have other questions, let me know.