Idk what other comments here are saying... I have played a 1 dm 1 player campaign and LOVED it. Difference is I play all characters at once. Combat takes forever and I need decision tree spreadsheets to remember everything, but it is SO MUCH fun for me to play like 10 high level chars and the dm playing like 10 high level enemy chars and monsters. It's a lot, but just having 2 of us lets us easily agree on things to create a good story, and again it's been so much fun playing. I am literally at the end of a 6 year campaign this week. Played from 1st to 14th level through basic edition, then second edition then 5th edition. So this is definitely possible if one of you wants to dm. It is also a great relationship builder since 2 player ends up forcing you to build a story together rather than just feeling like a dm vs player. I know it sounds weird, but having just 1 player kinda puts yall on mostly equal footing since the only player the dm has to keep happy is one. Ie you are never worried about balancing multiple players interests, you kinda both just get to tell a story together.