In some places, it is popular (or occasionally done) to say something like 'well, this area only has the money for 1/3rd of a sysadmin, and this area has the money for 2/3rds of a sysadmin, so I know; we'll hire one sysadmin and split her up'. It is my personal view that this is likely to be a mistake, especially as often implemented. There are at least two pathologies you can run into here.

The basic pathology is that humans are frequently terrible at tracking their own time, so it is quite likely that you are not going to wind up with the time split that you intended. Without strong work against it, it's easy to get pulled towards one side because it's more interesting, clearly needs you more, or the like, and then have that side take over a disproportionate amount of your time. Perhaps time splitting might go well if your one sysadmin is a senior sysadmin with a lot of practical experience at doing this and a collection of tools and tricks for making it work. If your one sysadmin is a junior sysadmin thrown into the lion cage with no support, guidance, tools, and monitoring, well, you're probably going to get about the results that you should expect.

The more advanced pathology is that you are putting the sysadmin in the unhappy position of having to tell people no for purely bureaucratic reasons (or to go over and above their theoretical work hours), because sooner or later one of the areas is going to want more work than fits in the X amount of the sysadmin that they are entitled to. At that point the sysadmin is supposed to say 'sorry, I switch over to area Q now, I know that you feel that your work is quite important, maybe more important than area Q's work, but I am not supposed to spend any more time on you until next week'. This is going to make people unhappy with the sysadmin, which is a stressful and unpleasant experience for them. People don't like inflicting those experiences on themselves.

(The actual practical result is likely to be either overwork or that once again the actual time split is not the time split you intended.)

I feel strongly that the consequence of both pathologies is that management or at least team leadership should be deeply involved in any such split-sysadmin situation. Management should be the ones saying 'no' to areas (and taking the heat for it), not sysadmins, and management should be monitoring the situation (and providing support and mentoring) to make sure the time is actually winding up being split the way it's intended.

(There are structural methods of achieving this, such as having areas 'purchase' X hours of work through budget/chargeback mechanisms, but they have their own overheads such as time tracking software.)

If you like, of course, you can instead blame the sysadmin for doing things wrong or not properly dividing her time or the like. This is the 'human error' explanation of problems and as always it is not likely to give you a solution to the problem. It will give you a great excuse to fire people, though. Maybe that's what you actually want.