A new member named Nicole asked this question (forwarded to me via Maggie N.).
In a sense, of course, all of WikiTree is one tree. We're working on one shared tree, so there is no my tree and your tree. But that's not the point of the question.
Sometimes a member asks if they can manage a second account for their spouse or a friend. This is tricky and it's not encouraged.
It's better if the spouse or friend has their own account. They will have their own profile, and the proper way to do it is for the profile to have an e-mail address associated with it and for the person to manage it themselves. That doesn't mean they have to do all the work themselves. You can add and edit profiles for their family members, it's just not recommended that you do it in their name. You should do it under your own account name.
A few WikiTreers do it. I've known a few people who sign-in as if they're their spouse and edit as them. Then they can manage a separate Watchlist. Sometimes the plan is to eventually hand over the account. But this really isn't recommended.