Apple's developer accounts have zero overlap at the moment.
If you are an iOS developer, nothing that you do on the iOS side changes if/when you subsequently enroll for Mac development.
The only foreseeable conflict is one it's unlikely to happen in practice. If you were running a beta version of Xcode for iOS and want to run a different beta version of Xcode for an unsupported beta version of Mac OS X, you might have to have two machines (or a dual boot setup) to do the beta builds simultaneously.
You should start with free: download Xcode and make an app for the shipping OS using the free SDK. You don't need to pay for a Mac developer account until you need to sign code or access pre-release SDK. Pay the money only when you hit a roadblock. Same goes for iOS, start free and only pay when you need to do something that the paid account allows.