This should be fine with such low power appliances. Just make sure the extension cables are decent quality and that the plugs are fully inserted.
Obviously make sure they don't pose a trip hazard.
Welcome to the forum!
What's your electrical background? Presumably you aren't expecting to go from zero to working in people's homes in 14 days.
Worth checking out some of the other threads on here with a similar theme. Not of good info and advice to be picked up.
I'd prefer at least a couple that are either from a combi or a pump. Better shower experience and less risk of big cables and overheating isolators etc.
Stick the two 4 ohm speakers in series and you won't damage your amp. Won't be hifi quality due to the more restrictive bandwidth of a guitar amp, but still may sound OK.
Or if the amp really is 2 x 4 ohm then connect one speaker per output.
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