The ones you mentioned are probably the best sources for verbal material (especially PR). Some say that Manhattan Review
does a good job explaining SC, I think they have a specific SC work book. The only way to get good at sentence correction is by doing hundreds of problems and thoroughly reviewing mistakes.
As far as reading comp., some people recommend practicing with LSAT passages. I have never tried this, but given that these passages are generally much longer than GMAT passages, I am sure it will help build up stamina and ability to stay focused.
Recently someone published a number of verbal questions, look under the verbal section.
With all this you should have plenty of study material.
As far as studying simultaneously vs. seperate, I don't think there is any right way to do it. It's whatever works for you. It's important that you maintain a balance and that you don't overemphasize one thing too much while neglecting to study another. Good luck.