You can add a person unrelated to the profiles that you have previously added. You can also link them by using a profile link in the biographies of each. This is done all the time. However, each of your trees (or saplings) will eventually be attached to WikiTree's one big tree, not necessarily in the current generations but further back in time with common ancestors.
You don't need another login to do this. The profiles of people in both of your trees will be in your watch list. If you want to have each tree in a separate list so that you can readily identify which person is which then there are 2 ways that you might handle this.
(a) Set up 2 free space profiles, one containing links to all of tree 1 and another containing links to people in tree 2. It will mean that whenever you add a new person you will have to update the appropriate free space profile. (http://www.wikitree.com/wiki/Free-Space_Profile
(b) You could also use a Personal Category (http://www.wikitree.com/wiki/Personal_Categories
) with 2 subcategories, one for each tree. Every profile you create will then have a category that defines which tree it belongs to.
On thinking about this I believe the category approach is cleaner and will be easier to maintain.