Plugging 1 into another will not give you more current.
The only way to get "more" current is to use 1 transformer for 1/2 of the gaer and the other transfomer for the other 1/2 of the gear. If you have a hum problem, while using these isolation transformers, then it is due to some interaction between the gear plugged into the transformer and you would need to experiment to find out what is causing the problem. There have been many discussion about solving a hum problem, review them.
The only way to get "more" current is to use 1 transformer for 1/2 of the gaer and the other transfomer for the other 1/2 of the gear. If you have a hum problem, while using these isolation transformers, then it is due to some interaction between the gear plugged into the transformer and you would need to experiment to find out what is causing the problem. There have been many discussion about solving a hum problem, review them.