Themeforest licence with use in MAMP pro

Do I need to pay for two licences when I purchase a wordpress theme and develop the website in MAMP Pro on my local host?

Nope! The license terms only limit a license to a single end product, that of course would include testing of the end product.

It should be noted that some authors choose to include their own licensing system in their themes. In these cases you may want to update the theme manually on your MAMP server, and only activate it on the production server; but, if you do want to activate both, you’ll need to get in touch with the author if their system prevents you from doing so.