That is OK as there is no crossloading effects with separate transformers . Just verify the common input and output connectivity ( i.e. polarity if diode connection to ground) to avoid internal short on the ones that you pair.
He means the same as I. f all the audio equipment uses a bridge rectifier with diode drop to ground, you have to guess which side is grounded."
AC+ or AC- <> diode<> Gnd
Here is a picture of two units, one with a negative voltage bridge on top and V+ bridge in bottom unit. When both share the same the transformer output, It shorts out then creates a large voltage when removed. Can you see the short?