Q:

Are parallel branches the best way to manage similar projects which share code?

I have two projects (Product-line A and Product-line B) which share about 90% of the same code. I want to migrate both projects to GitHub. 
Should I use the common code as the master branch and create a new branches for product-line A and product-line B? Would there be any disadvantage to having two parallel branches that I have no intention of merging? The code will need to be used by multiple users from multiple accounts.

A:

Would there be any disadvantage to having two parallel branches that I have no intention of merging?

No disadvantages: branches aren't always meant to be merged.
The only issue with that model is to clearly differentiate commits/changes done on the common code from ones done on PA or PB: changing files in PA or PB alone is not enough: you need to check if those changes are part or not of the common code.

