1. They are more of a business/school friend. You share knowledge and passion. You compliment each other's strengths and cover each other's weaknesses. There is little risk to forming a mutual business partnership, since loosing such a friend is part of the game.
2. You were meant for each other, perhaps you are even married. This venture will make you even more entangled, increasing risk but also reward to your relationship.
Be VERY sure if you choose #2. Strongly consider whether your relationship can handle the added risk of loosing the business and consider when it's time to walk away from the business (despite the financial consequences) in order to save your "marriage". In my marriage we've made bad financial decisions (wrong house purchase) where walking away cost us 100k but I think definitely saved the marriage. Loosing that money was worth every penny.
For #1 it's more of a straightforward business decision.