I did something like this for many years, back when Amex UK had CX Gold (== oneworld Sapphire) as a platinum perk. I had my BA (blue/bronze) number on all bookings for avios/tier point collection but used the CX card to enter lounges. I only once had my FF# forcibly changed by a lounge agent who insisted it had to be that way... and I promptly changed it back on the Finnair website once I was sat down with a drink!
The technical industry terms here are FQTV and FQTS:
perfect, thank you so much!! Sounds like it’s definitely worth giving it a go - like I say, would only help with the check in queue since I don’t have lounge access 😢 but will be helpful if the lines are long