You could probably mix less aggressive fish from each lake such as labs or peacocks with something less aggressive. As far as plants go I'd decide what fish you want first and then ask again on here about plants. Some cichlids will dig them up while others won't.
The ever popular Labidochromis Caeruleus (Electric Yellow, Yellow Lab) is by far the best beginner fish from lake Malawi. They're hardy, tough, not too aggressive, and when poor breeding hasn't destroyed them they're very attractive.
Technically the two lakes can be mixed, but it's easy to mess up and best if you don't. Malawi's do best with Malawi's, Tanganyikans do best with Tanganyikans.