I use a pair of the push box calls. You can hold them both in one hand and work the push sticks with two fingers. This way it sounds like two birds going at it. This can also work very well in the spring. There are push box calls that have two sticks in one box available (Knight & Hale, I believe) but I've used my pair for years and am satisfied with them.
I've bought the Knight & Hale fighting purr push-pin calls...one sounded decent the other didn't. Then I got the one with two sticks in one box. Thats a Quaker Boy call. One side sounded decent the other didn't.
Then it finally dawned on me that I could just do it with a pot call. Slate or glass IME, glass for a louder fight purr.
I always have one with me, in fact most often its right there in my hand if I feel I need to use the fight purr call. Creep up on your striker just a little to get more purr out of it. If you have a mouth call in, you can also do some cuts and putts along with the fight purrs if you mant to sound like several turkeys.
No need to carry around another bulky call and then have to dig it out with turkeys nearby if you want to try the fight call.
I can easily vary the loudness from very loud to a quieter fight purr.
I just purr aggressively in a sharp V to get 2 slightly different tones. Not even sure that is needed as I think the two tone thing is a gimmic the mfg's want you to believe so you will buy their product. I've asked the turkeys if you need two different tones, and 5 longbeards, a slew of jakes, and a few hens have told me "nah, you don't need that".