I take lessons once a week (Weather permitting) my instructor charges 20.00 for an hour lesson. She's really good, too. But in your circumstance maybe you could go to both: one instructor one week or two and then the other for the same. That might help.
When I started with my current coach she requested that I cancel my privates with my other instructor. At first I was a little iffy about it but after having 2 lessons a week, each with a differant coach I realized I needed to drop one of them. It is very difficult to have 2 coaches teaching you differantly in the same week, every week. So my current coach is $50 for a 1 hour private if you use a lesson horse. I believe in your own horse its $35-40. And it is definitely worth the cost. In 2 years she's gotten me from always getting the wrong canter lead and diagonals to 99% of the time if not always getting the right ones. She also got me to place at every show in my first show season. So the fact that she's expensive is definitely worth it. I believe privates wit her are $75 for an hour. Done with my rambling I do agree with everyone. Keep on with your first instructor, the one you know is good. 2 great lessons a month are better than 4 mediocre ones.
Charis makes a very good point and so does Beau Baby. It is difficult to have 2 instructors teaching you. If they have different teaching styles it can be confusing. I would go with the more expensive instructor because she seems to be worth the cost.