Find what is determining whether it is running already, and make a virtual file for it.
Most of the time it is handled by a mutex. You can find these by using a tool such as Process Explorer, selecting the game process and pressing CTRL+H to view the handles. Scroll down until you find the mutants/semaphores. The most likely one will be a mutant, although be aware that there are some mutants that are used anyway, so most programs will already have 3 or 4 of these. The trick is to figure out which ones are the standard ones

Standard ones are like \sessions\1\BaseNamedObjects\blahdeblah.
If it happens to be detecting an existing process by executable name, that means you'd need two copies of the executable. This is fairly rare though.
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