RAM/memory based databases
I see two competing/conflicting requirements:


  RAM/memory based database


and 


  more than one executable to be able to access the DB concurrently


They are competing/conflicting because modern operating systems normally enforce strict segregation for RAM allocated to specific processes (there are exceptions, but in the main this holds). 

One option here is to copy your database to a RAM disk. That would allow separate apps to treat the database as if it were a file. The downside here is that the contents of a RAM disk won't survive a reboot of the host machine, and so you need a very robust system to keep your database saved to a real hard disk as well.

But really, if you want multiple apps sharing the same db, you're much better off using a database server. Enforcing safe, concurrent access is one of two main reasons databases exist as distinct entities from flat file stores (the other is efficient joins).