Thanks Mike!
I suppose I do benefit from this the other way – if I touch in before 0630 in Maidenhead I can get all the way to Canary Wharf off-peak, even though I’m taking peak time underground services.
Since this is my frequent commute the 2-card method is probably optimal as it allows me to get straight on a GWR fast service at Paddington. Interesting to know the other methods though!