Hi there
I guess 180 degrees will not solve your problem, you'll need to adjust these two sensors on a different angle, lets say 45. So how is this going to work:
Make one of these sensors a primary one, what I mean is calculate the time it takes to move from this to another. Mark/select the sensor from which you want to start your timer. As wheel moves in one particular direction, lets say in a way that the timer value comes out to be 1 second. So when it does move in an opposite fashion, the timer value would be around 3 seconds or so (theoritically). This will happen due to the difference in angle, from either directions i.e. in one way its 45 while in another it is 135 degrees. Also try out getting the mean of some set of consecutive values, this will ensure efficiency.
Hope I have suggest a suitable way for the same.
Good luck