I am using the two Nux pedals together as described earlier and the sound has superseded anything I've heard from similar kit on which I invested thousands of pounds on in days gone by. They have cost a total of £80 and are worth ten times that amount.
Like the post mentioned above, you can use almost any device as your fourth head and as long as you get the feedback and levels correct it will sound good. I've also tried it with the three heads on my Varispeed Copicat with a Nux pedal set with a louder fourth head and it's excellent. I have to remember not to use REGEN or feedback on the first three heads, or at least very little.