In control flow, drag the green arrow to the email task, then right-click on it and you will see you can set it from 'Completed' to 'Conditional', then you can set an expression on the condition. The arrow will then turn blue. You should then be able to drag another arrow to the other script, and set that to conditional.
I have this set-up often, many times you want to email if a certain condition applies. The standard syntax for the conditional constraints is something like:
@[User::SendEmail] == True
Assuming your SendEmail variable is a boolean. If you use anything else, just construct an expression that evaluates to either true or false.
Remember to set the conditionals to OR instead of AND, otherwise it won't complete unless it can take both routes!