I can suggest you a simple trick I've used, easy and free of charge.
Use a software called TX-line (download it from AWR, it is free).
This tool has different line model to synthesize and/or analize transmission lines.
You have just to put your microstrip parameters in the first tab to check electrical parameters, such as impedance and so on. Using the same phisical parameters in the Couples MS Line, you can play with Gap until you'll see that the impedance is not affected any more. This means that the coupling between the two lines is negligible.
I hope it can help.
Mazz