It's not as simple as a shared variable but the two add-ins are distinct programs and don't share anything. However there are potentially a couple of solutions for passing information between two add-ins.
The first is for one add-in to write information into a location that the second add-in has access to. For example, you could create an iProperty on a document or create an attribute. The problem with this approach is all of the locations where you can write custom data are in documents so the second add-in would have to know which document to look in and you would be dirtying the document which could be confusing to the user.
The second approach is that an add-in can support it's own custom API. For example, one of your add-ins could support a public method called SetValue and the other add-in can find the first add-in and call this method, passing it the value.
A third approach I just thought of is to write the information to a file on disk. This is similar to the first solution but has the advantage that it's not editing anything in Inventor. It would also be simpler to implement.