The T-Rex Engineering Polyswitch AB is a really cool and convenient pedal that is designed to allow you to split signals in a variety of ways.  It is like a traditional AB box where you would be able to choose between two different outputs, but this has a few sets of inputs and outputs.  The pedal has two 1/4 inch inputs and four 1/4 inch outputs and it splits up the outputs into two sets.  This is a great pedal to have because it cleans up noise if you are using two different systems and allows you to indeed use the two different systems simultaneously without having to switch anything around.  While this is a nice pedal to have if you are have a large set up, it isn't necessary and really is only a luxury for those of use who have the resources.  I will say that this does do it's job quite well and is built like a rock.  I don't own one of these but I have used it a number of times in my friend's set up.  Because of the steep price, I can't afford to have one of these in my set up, but if I did I would definitely pick one up because it gives you a lot of options.  The T-Rex Engineering Polyswitch AB will give you more options for routing things and in the end will give you more sounds.  The only bad part about the T-Rex Engineering Polyswitch AB is the price as they make it hard for most interested parties to get.  Beyond this, I think that this is a great pedal to have and if you can afford the T-Rex Engineering Polyswitch AB you should definitely look into it!  If it were only a bit cheaper I would imagine that more people would be interested in this pedal...