1. How good the other party is when it comes to collaboration. Some coders work well with others, and some are lone wolfs.
2. If you have complimentary strengths: One programmer is strong at X and your string at Y.
3. The energy from both programmers makes each party proactive and thus avoid procrastination.
I also think there are the larger questions of if you're using a spec, etc.
1. How good the other party is when it comes to collaboration. Some coders work well with others, and some are lone wolfs.
2. If you have complimentary strengths: One programmer is strong at X and your string at Y.
3. The energy from both programmers makes each party proactive and thus avoid procrastination.
I also think there are the larger questions of if you're using a spec, etc.