That's not as easy as it would appear. I'm hoping this is a very scaled down look at what Macro A & B do. If they are small, copy and paste the code if possible.
I'm guessing they aren't small. You can use includes statements to "read" macro B into Macro A, then pass the variable just like to a normal sub.
Another option is to write the value to a text file in Macro A before calling Macro B.
There are also some API calls you can use I'm sure, but I don't know what those are. If you need to use API from EB, you should probably be in a better language.
Good luck,
calculus