I never understood betting odd, like 25/1. I assume it means for every 1 dollar you bet, if you win you get 25 times that back. If so, then that is what you use for your expected value:

Probability Both Horses Win - 0.06

Probability Both Horses Don't Win - 0.94

To calculate expected value we multiply the probability of success, by our random variable's 1 and 25 respectively:

0.06*25+0.94*1=EXP for every one dollar you bet.