Depends on what you want to do, how you're using two amps, and if the amps have effects loops (and if so, series or parallel). Almost anything's valid as long as the levels are set correctly (per user manual). The most important thing is that it sounds good to you.
Guitar >> TF In A >> TF Out A >> Amp 1
………………………TF Out B >> Amp 2
Guitar >> Amp 1 >> FX Send 1>> TF In A >> FX Return 1
……..>> Amp 2 >> FX Send 2>> TF In B >> FX Return 2
Guitar >> Amp 1 >> FX Send 1 >> TF In A >> TF Out A >> FX Return 1
……………………………………………..>> TF Out B >> FX Return 2
Guitar >> Amp 1 >> Direct Out 1 >> TF In A >> TF Out A >> Amp 2
Just a few examples of setups I've used.