With running parallel, you mean that there are two different processes running, e.g. you're starting both .exe-files independently?
If you're doing so, you have to use mechanismns of Inter-Process-Communication.
Have a look
here or google for e.g.
c# inter process communiation.
If you don't have two exes running, you could take usage of a static variable in one of your projects. Take any (public) class of project 1, declare a public static variable in it. Create a reference from project 2 to proj 1 and access the static variable via it's namespace, class and name.