RPs are cheap, really, when you compare the cost of an RP to the cost of a dedicated effects device that only does one thing. There are plenty of guitarists out there who use more than one effects pedal, and there’s no reason why you can’t do the same with two RPs.
All you have to do is run the output of one RP into the input of the other. Now any sound that comes from the first RP in the chain will be processed by the second RP in the chain.
If you’ve got two of the same device–say, two RP255s or RP355s–you can use the first one to run all the amp modeled patches (say, from my version 14d patch set), and the second to run the FX-only patches from my version 15 FX-only set. Now you’ve got your amp modeled patches and effected patches running into a whole ‘nother effect.
This works great for stuff where one patch just won’t do it. For example, let’s say you really like one of the low-octave patches, but you wish you could run it with a rotating speaker effect for an organ sound. With two RPs, you can put the low octave patch on the first one in the chain, then put the rotating speaker effect up on the second RP. And you’ve got footpedal control over every effect in the chain.
If you do this, it’s a good idea to put any reverb or delay FX on the second device in the chain, not the first. Things get muddy pretty quickly when both devices have reverb or delay on them.