Charter flights are conducted in Robinson 44s and they've only got the one piston engine. However, the operation has to be through an AOC so it's a matter of whether any particular operator wants to have SEPs on their AOC. Some may consider that there wouldn't be enough revenue generated to make it worth it.
Trial lessons are training flights surely? If they are a pleasure flight, then there would be no opportunity for the passenger to have a go, the dual controls would be removed and the pilot needn't be an instructor.
Cheers
Whirls