I suggest you only allow them access to 1 sheet each in a separate workbook, and then consolidate the entries on a sheet in a 3rd workbook. (This method can be expanded to add extra sheets in the book if input from more users becomes necessary.)
By that i mean, user 1 enters into book1/sheet-user 1, and book2/sheet-user2 is used for the 2nd user. How much space on each sheet is upto your requirements. (you can do the normal restrictions then for each book.)
Then to show each user entries on the same sheet, paste the links to the columns on a sheet in a common read only book.
Not knowing exactly how you are intending to use the data entered, this solution might seem vague. I am hoping it might be enough to give you a nudge to ward the solution you are looking for.