The fine looking gent on the left is Brutus. He has a white patch on his nose and the white portion of his tail is a bit longer. The only criteria that was required of Brutus was that he be visually distinct from Casper. As it turns out he is far more cuddly than Casper - and very easily led astray.
The kids can tell them apart - but it's cheating in the way they do it. Brutus has a black collar - and B is for Brutus and black. Casper has a purple collar - and P is for CasPer and purple. It works - although Hayz still likes to call them 'dogs' or 'boys'.