Designing for 2 platforms at once?

Hello all. I am designing a came called "Psi" for, at least, PC and mobile devices. I have asked this question on one of my threads before but I would like to ask a larger audience. In your opinion, would it be better to go ahead and work on a build for 2 different platforms simultaneously or would you break the development down for each different platform and why? Thank you. I am loving this supportive community and am thankful to all of you for your opinions and help.

livingartgames.blogspot.com

If I know for sure that I'm making something for mobile then I build it with mobile in mind because the gameplay is going to be that much different to PC. If the mobile gameplay doesn't work as well for PC then I can tweak it afterwards.

The way I've done my game is to have two groups (Computer and Mobile) and to test at the start the game is on a mobile device, make group Computer deactivate and Mobile active, and if on Computer make group Computer active and Mobile deactivate.

You can do this through either using the Touch or Is Mobile detection.