Since there is not a 100% chance we beat the Aggies, the odds of beating both are smaller than just beating UW.
In this case, the chance of beating UW = .087 (8.7%)
Chance of beating USU = .453 (45.3%)
Chance of beating both = .087*.0453 = .039 (3.9%)
Was that your question? Granted, I do agree with you that if we beat UW then the chance of beating USU should increase to >50%, but the same principle applies.