Conceivably this is a better question for the Stack Exchange Security site, but...
The fact that you have encrypted the number doesn't prove you aren't cheating. It just proves you encrypted the number. The question you are asking is called, probably among other things, Mental Poker. The idea is that every party needs to perform some random step and encrypt it.
I included the reference because it can get you started, but for a single random number one might do:
- Alice picks a random number A from 0 to 99 and encrypts it.
- Bob does the same for number B.
- The "real" random number is (A+B) mod 100. This can be verified when both parties reveal their private keys.
You can use any single-key encryption for this simple scheme. You will have to generate lots of keys so they must be created using cryptographically strong key generation.
This scheme isn't computationally cheap.
You can probably do better for a specific algorithm, but hopefully this answer can get you started. I don't think any secure scheme will be all that cheap, but it isn't clear how many of these numbers you need.