Don't forget that, if each user is on the same server, the "load" on a server is the same whether it is spread amongst more than one user or not, and "too much load" is still too much load.
As is discussed by DH Honcho Michael in a previous thread, attempting to "distribute" load in this way is pretty pointless. You are not really distributing anything, like you would if the users were on different servers. Rather feebly obfuscating the party responsible for the overloading, as Michael points out, it doesn't really hide who is doing it, and it still damages the server and will have to be dealt with if the load is too high.
There are no longer any "hard" CPU limits per se; if your activity is negatively impacting other users some action will be taken, and whether it is one or 50 of your users (under your account) causing the load is really irrelevant - the problem will have to be addressed.
That said, I think there are still legitimate reasons to use multiple users (particularly script segregation, email quotas, and file-management/permissions issues), but trying to enable higher overall CPU/connection utilization isn't one of them. While it might appear to "work" for a while, you are only shooting yourself (and everyone else on your own server) in the foot - the server will be overloaded, everyone will suffer, you will be identified as the cause, and you will be right back where you were.
Some sites just can't be run on a low-cost shared host because they are shared; trying to game the system to run them anyway is naive as the system will not be able to handle the load whether or not any individual user produces all the load.
It's not like you can say, "Hey, which of my users violates your limits?", because DH makes no representation that you can operate up to a given "hard limit"; instead they say they will curtail or in some way manage you if you overload the server.
It sounds to me like you might be a perfect candidate for the PS service DH just announced. It might meet your needs, and is cheaper than a dedicated box.