Been there, done that, works well.then a simple and cheap way would be to use two (identical) 240V to 6V transformers - put the primaries in series, and the secondaries in parallel - job done!
You ought to try it with three 1KW isolation transformers in parallel, for temporarily feeding an immersion heater - identical transformers, wired them identically - BOOM!!! - one of them was wired differently internally. Then you've got the problem of finding out which oneBeen there, done that, works well.
But you have to get the phasing correct, otherwise the transformers will be destroyed.