Two PCs needed, two PCs accessible, only one is used by them! :(

Hi @Plushybunny - In TS4, you can lock just the computer to other sims (instead of locking doors). That should work for you and still keep the immersion. It's pretty easy to do.

Make sure your active sim is the one you want assigned to the computer. Then click on the computer > Computer Security Settings > Lock Computer. You have 2 main lock choices.

Lock Computer for... lets you choose to lock for "Everyone" (don't choose that one, no one will be able to use it) "Everyone but SimName" (this will mean no one else, including visitors or employees, can use that computer), Everyone but Household (not your goal here, but good for when you have random visitors show up and go straight for the computer), or Everyone but Employees (in case you want the butler to have his own computer, I guess) 

Lock for Specific Sim... gives a choice of all the members of the household. Your active sim will be able to use the computer, but so will employees, visitors or any household members you add later. 

Your writer might be getting up and going to the other computer if you asked them to continue writing something they started on that computer. I have had this happen to me before - for some reason, it seems to happen with programming and writing. I don't know if you can get them to finish on a different computer if you lock them out of the place they started. I know that I've tried to get a programmer to finish a video game at a different computer when the one they started it on was broken or sold and it didn't work very well. It's probably not supposed to do that, but you could have them finish first or scrap the book and start over on the assigned computer if you run into problems.

I hope this helps - Happy Simming!