Hello and welcome to MrExcel!
I would recommend splitting the form into two by having two separate forms. You could then recombine both forms back into a single form, by treating both halves as subforms on a third form (using formatting techniques, you could make it appear to be exactly the same as the existing form). It is this 3rd form that would have the Header etc. The reason for splitting the form is to give each half its own data source. I suspect both halves have the same data source hence you cannot log a user out until the other user has completed their signing in process. So you may want to create a new query to support the right hand side of the form. I haven't tested my suggestion so this is based purely on guesswork, but hopefully it is on the right track.