Can two male rats co-exist in one cage?

Generally only if one or both were young when introduced. Be sure the cage is just-cleaned so it does not have the one resident's scent too much. Make sure each has a retreat in opposite corners. Also put a little vanilla or non-toxic smelly or deodorizing substance on both rats to kill their natural scent, thus they cannot tell who is who or even who's cage it was. There might be a little scuffling at first, but after one rat has established he is the boss, both should get along fine.
Older males can be introduced, but its best to use the deodorizing method and realize that the two rats may fight anyway or never get along..Watch them carefully. Males can even be kept in larger groups if the cage is large enough. Do not add females to a cage with more then one mature male, as this will trigger territorial fighting between males.