2 active gateways on membership plugin

In the documentation PDF, it is said -

Note: Only one gateway should be used at a time! The only exception to this is the “free
subscriptions gateway,” which can be used in conjunction with paid gateways.

Why this restriction? I have seen many sites that offer a wide range of payment gateways to be chosen from.

I wish to deploy 2 gateways simultaneously. Users registering for premium content access on my site will need to be given an option to choose the payment gateway between paypal and some other. How do I achieve this?