I am yet to see a single trainer game with NTR. Remember, NTR, meaning "netorare", describes works (stories, manga, anime, games, etc.) in which the girl leaves the boy altogether, of her own will, most often because she doesn't fall in love with him, regardless of all the effort he puts into winning her affection (
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). I've seen the term abused and distorted many times here, without really understanding what it means. The truth, however, is that neither training nor brothel or slave management games are even close to NTR.
Sharing, on the other hand, I see as unavoidable in trainer games unless the training is very narrow and limited. I don't mind sharing my girls, as long as at the end of the day they come back to me.
JaxMan's proposal does make sense, I think, so if it would satisfy you, I'm all for it. An option to abstain from that and just let the training proceed without you would be good, however, as your presence during the training requires you to miss a day, and you might not want that.