First of all, sorry if my answer sounds rude, but I prefer to be realistic instead of condescending.
As I have little programming experience I need to outsource this but I
have to have it done within a week.
Most reverse engineering projects, even when done by seasoned reversers, cannot be done "within a week", unless they are trivial.
If I choose two freelancers in say two countries to speed things up how
can I make sure that they're not overlapping each other and the
disassembled code is correct?
Short answer: you cannot be sure. Also, most likely, having 2 people working on the same database will slow down it instead of speeding up. It might speed things up when working on different, isolated, pieces of code/binaries and, still, communication between different time zones, agreeing things, explaining what one did to the other, etc... will slow down the project.
What systems are used in such cases? IDA pro seems to be the most
popular choice for this MCU
IDA is the de-facto tool for reverse engineering, although other tools might be used. But, in any case: reverse engineering an ECU (probably) by 2 persons within a week? Yeah, no. I don't think it will work unless you find out people that already have done that. Good luck...