Multiple instance properties

I'd like to understand how tuning works on instances that are living inside the same machine/VM. I couldn't find a straight answer to the question if properties of each separate instance have some impact on hardware specs of the machine. Here's what I mean: take a server with 8 cores and 24GB RAM, for example, running 2 instances. If I set the max server memory to 10GB for each instance, Cost Threshold for Parallelism to 50 (to enable/set better multithread) and Max Degree of Parallelism to 3 cores, does that mean that both instances will use * 3+3=6 cores so 2 cores for the OS? * 10+10=20GB so +4Gb for the VM? * Cost Threshold for Parallelism so be set to 25+25 ? I haven't found anywhere this kind of info and it would be nice to know this for better customization of instances I currently run, and for future reference as well.