This is absolutely fine, and something I often do myself. The fare charged is based on the touch in time at the start of the journey, not the service you travel on. All a RPI will be interested in is whether you've touched in.
At the end of the peak period it works the other way round so someone touching in at 18:55 intenting to catch a train at 19:01 is charged a peak fare.
I don't think the OP's question wasn't about whether someone touching-in 'early' (before the peak period begins) itself was legit, more about whether them
also touching-in their partner's Oyster card
without their partner being present at the time was legit...
Hypothetically, I wish to take the 16:15pm from an ungated station. I touch in both my and my partner's NR-discount Oyster cards at 15:59, saving approx. 50% on the fare. She arrives at the station at 16:10, after I already tapped her Oyster card in. Is this fare evasion?
In practice, at an ungated station, this would only be an issue if there is an RPI 'blockade' of the stations entrance (or entrances) - and if there was, then the OP would want to stay 'outside' the fare-paid area so as to handover their partner's Oyster card to her so she can travel.
(Though depending on how stringently the RPI blockade is set up, it might still be possible to make it work - but attempting to do such a thing in that scenario could legitimately arouse the suspicions of RPIs. Perhaps best in that circumstance be for the OP to touch-in and benefit from the off-peak fare, then reappear at the blockade line when their partner arrives at the station to handover her Oyster card so she can then touch-in, albeit for a peak fare.)