“ If you can meet with Triumph and Disaster/And treat those two imposters just the same
The poets decision to capitalise, implies more that that he wanted reader to play close attention to these two words. In english grammar, only proper nouns are capitalised. His use of ’meet’ supports this interpretation.
DISASTER = discourages people and make them give up on noble pursuits
TRIUMPH = convinces people that they no longer need to improve themselves
these two are imposters as they both have negative effects