I want to make a machinima in Halo CE and I have some questions

Is it possible to control 2 different people by yourself on 1 computer? Like on an xbox you can if you have multiple controllers

in devcam, can you stop the camera from resetting when you die

for the first, you cant but you can try to script it, but it wont look the same as you intended

for the second, no, but you can always spawn a biped that looks the same as your character and kill it.

No you will need two seperate computers to actually control two bipeds. That was how I filmed most of the Halo CE Chronicles, two computers next to each other.
No. But as the previous poster said if you place a biped that looks like the character then you can kill it and film it with the dev cam.

There are other tricks you can use such as creating or modifying maps to place AI doing repeatable things: walking running fighting, placing bipeds so that the server operator can bump possess them (take them over) add or move scenery objects...etc. Having the ability to modify the maps give you much more leeway to create the scenes you may desire.