You can do that on Windows with out of proc COM servers. That is mainly used for embedding documents in other documents (e.g. having a spreadsheet displayed in a word processor document).

Well the simple answer is no then. Less simple answer is you would have to call functions across process boundaries and you would probably have to reverse engineer the exe you are calling into to find the addresses and data structures you need.

There is an entire industry dedicated to RPC (remote procedure calls) and the technology has been evolving for decades. Tech like "SOAP", "COM", "CORBA", "EJB" and now "API" are terms you can use to search for this. That's what those buzzwords mean.

If you can guarantee you're going to be running on the same machine using the same architecture and runtime between EXEs, it can get a little easier. This is exactly how a lot of infection viruses and trojans work. In the case of games doing this, an excellent example is the DwarfTherapist tool used as an adjunct to Dwarf Fortress. It's Free software, so you can download and study how it works. It doesn't actually make function calls (Dwarf Fortress itself is not Free), but it effectively has the same effect.