For the most part, that's a very bad idea especially if you want to keep two African lungfishes or SALs together. AULs can be kept together, but that's really the only 100% safe combination; all other combinations will probably result in injuries/death to the lungfishes from fighting.
With that being said, there are a few cases of lungfishes other than AULs being kept together in relative peace, but these are few and far between. King-El can elaborate more on this subject.