raphaz You only need one transmitter and one receiver for 2 speakers as long as you have long enough cables to go from the receiver to each of the speakers. One for the left channel and one for the right and you can place the receiver somewhere in the middle.
You may need adapter plugs to go from whatever type of output your source has (XLR, RCA, 1/4") to the transmitter, as well as whatever type of connections your speakers have to connect the receiver to the speaker's inputs. Those are cheap!
As far as apps, there is no app. They are independent units that communicate at 2.4 GHz from the transmitter to the receiver. Similar to wirelessly connected microphones, guitars, etc.
I don't have an opinion on Wiim Mini. I've never messed with those, but I can tell you that there is no substitute for full size audio components if you are concerned with fidelity. I like all my stuff wired!