Okay, that's a bit clearer. Control.Parallel is not what you want; that is for parallelizing pure code (and is very nice inside its little domain). But you want concurrent code: multiple threads that change state at the same time. That is in Control.Concurrent.

In particular, the "shared variable" is Data.IORef.

Simple example from which you should be able to extrapolate.

Apologies, this code doesn't compile. Here's the fixed version:

import Control.Concurrent
import Control.Concurrent.MVar
import Data.IORef

threadBody var = do
    x <- readIORef var
    writeIORef var (x+1)

main = do
    var <- newIORef 0
    -- spawn two threads
    forkIO (threadBody var)
    forkIO (threadBody var)

    -- look at the value of the variable
    x <- readIORef var
    print x