While it's certainly possible to mix the two, the best thing you can do for all the fish involved is to choose either one or the other. I'll try to give a little summary of both and perhaps it can help you choose a decision, since there is much more to these fish then appearance.
Malawis are the perfect cichlids for beginners. They are very hardy, flexible with water perameters, and simply a lot of fun to watch. You simply can't get better color assortment in fresh waterfish, and they do great in higher numbers - adding up to just about the nicest and easiest freshwater show tanks one can maintain. A nicely rockscaped mbuna tank will produce a 'wow' factor from visitors that no other freshwater aquaria can.
Tanganyikans are more of a connaisseur variety of fish. They are extremely diverse in shape and behavior, but tend to lack in the color department. These fish have a ton character, and unlike Malawis they settle in breeding pairs or groups and establish solid territories. Because of this the tank will be more sparse then a typical malawi tank as well. These fish are more for the hobbiest and won't produce that 'wow' factor from guests that a Malawi tank would. Tanganyikans tend to be less tolerant of soft water, nitrates, and high temperatures, and thus require more diligence in maintenance then Malawis.
Either way, aggression is a big concern, and the safest thing to do is assume different species are not compatible until proven otherwise. A lot of great reading has been provided, and once you decide on the direction you want to go, make sure you do research and get advice on the species that interest you, both to make sure they are compatible and to ensure that you provide them in appropriate numbers and gender ratios. It's much easier to spend time in advance planning the setup, then spending even more time, not to mention money, fixing problems later on.
Cheers.