Managing one site with multiple devs?

Hi there! New web dev here. Me and my fiance have been wanting to start a website for a little while, and we finally made a neocities account. The problem is, he seems to be able to log in just fine, and I cannot. Same password, same URL, but nothing seems to work for me. We both want to be able to contribute to the site in our own time, so how can we both log into the neocities account and work with the code at the same time? Any pointers?