you only need one. I got mine for $240 from Amazon shipped 2 day. I absolutely love it. I still do most of my mixing on my mains, but when I am struggling with a mix decision or just want to see if it's sitting as well as I think it is, I flip over to the MixCube. Usually I am wrong
It's great at finding that mid range detail you are missing on your mains for some reason. It's also kind of cool that the act of switching to it kind of hyper focuses you to listen intently.
If you get those Tascams, only use one. That's part of the technique with the little grotbox is to see how your phantom center falls apart in a mono mix which is truly what most people will hear. So many people are listening in headphones that aren't seated very well, through systems that have speakers at diff heights or just oddly off axis, through TVs, in cars that have you far off to one side of the stereo image. The mixing through one makes you focus on your mix translating in that type of situation.
Also, it being sealed is supposedly one of the points also. It's going to give you a realistic representation of what low mids are there, and what normal highs are there.
That's where you will find you stuck all your vocal breath too high and that you put your kick sub so low you can't hear it now.
It's all I use now for settling in vocals, snares, kick drums, guitar solos, etc. The key word here is balance, you'll instantly hear what is out of balance, even if it sounded ok on your main monitors, you'll be surprised by how "that vocal is way to loud!" or "I can't hear the kick drum at all!"