(As well as the connection to the Focusrite Scarlett 2i2, you will have to decide where to attach your monitor speakers.
There is no way of using a single 'stereo' cable. In practice, I can confidently state there will be no discernable difference. The balanced option has theoretical advantages. If you use a 2-conductor cable with TS jack plugs you will get an unbalanced one. If you use a pair of 3-conductor cables, with TRS jack plugs, you will get a balanced connection. Your Focusrite Scarlett 2i2 has combo jacks, accepting a microphone signal on an XLR plug or a Line input on a TRS jack. But we don't need to worry about that for now.)
(The outputs are duplicated on a pair of balanced XLR sockets, which carry an identical electrical signal. Your yamaha mg124cx has its stereo main outputs arranged as a pair of balanced TRS 1/4' jacks. Now you've asked a specific question, you can have a specific answer!
