I don't think you can do what Malinko is asking in Excel. S/he's asking about being able to type in EITHER cell. With a linked formula, that only works if you do all the entry in ONE of the two worksheets. It also gives you no flexibility if multiple entries in one workbook get added into a singly entry in another, particularly if those entries are not fixed in number. It would also be very cumbersome to do with multiple entries for multiple employees, because you'd have to manually create the formula for each field in the master sheet.
Malinko, why are you using Excel for an application like this? It sounds like you need a database software where you can define fields from individual database entries all be linked to a master record.
Another possibility...do they need their own sheets, or could they modify a master sheet? If you just need them all to see all the entries, and you're using MS Office, then have you considered using MS Groove? It creates a collaborative environment where you can "publish" a version of a file for the entire group to use and every time a member of the group modifies it, it automatically changes everyone's copy of it. I haven't actually tried it myself, because nobody else I work with has the same version of Office I have (this is what I get for getting the newest version when everyone else is clinging to the older version they know how to use), but it looks like something that would be useful for such a situation.