If I did not know what horse you are talking about, this would be a difficult question to answer. The first matter to address is “Are you being fair to the horse?” If she has very limited turnout with no opportunity to exercise then she deserves to have more than two chances a week to get some attention. Whether it’s you or someone else, whether it’s riding or lungeing or grooming or handling, that doesn’t really matter. It’s not keeping her cooped up that’s important.
I can also think of some horses that are simply too difficult to ride if they’re only worked infrequently. While some come out of the stall after a long weekend off as comfortable and settled as the end of their last ride, others react badly. If too much time off makes your horse anxious, nervous or so strong that your ride is unpleasant, well, who needs that?
Another issue is “What do you do with her on the two days that you do ride?” Vets whose practice is in foxhunting country well know the colics and “tying up” they have to treat on Sunday night in horses who were hunted hard the day before, having not been kept exercised during the week.
And then there is the whole question of the rider’s expectations. You can ride your horse a few times a week and trundle off to a schooling show and probably survive. If you get out of breath or find your horse is late to your leg or very distracted, you should not be surprised. If those things don’t matter too much, then have at it. But if you pay for a professional to school your horse once a week and you only ride her one other time, you’re not exactly setting yourself up for success in First Level at a recognized show. That doesn’t mean you can’t do it. It does mean you have absolutely no business being disappointed or upset with your horse if the outcome leaves something to be desired!