The only way to use the MixingLink as an A/B footswitch is between the Mic, Line or Instrument as the A input and the FX Return as the B input. The DI/Line Out has the same signal as the Amp Out so If you try to loop the DI/Line Out to the FX Return Input you will get feedback and potentially unpleasant sounds.
The From FX Input Impedance is 200k vs the 500k impedance on the Inst Input. If you are planning to connect a guitar as this "B Input" you may loose a bit of tone, but it depends on the guitar.
