I would think that you should separate your game - spend as much time on your doubles as you would do on scoring (whether that's aiming for the sixty, or T19's or whatever). Alternate it as well - if you have a spare two hours of an evening, do an hour of doubles then an hour of trying to hit ton+'s.
That's only my advice, and I'm sure that all the guys on here are a damn sight better throwers than I am - see what they say!