Usage instructions for using both PayPal and Stripe

Our Pro Site installation has been up for 2 years using PayPal as gateway. We have had many, many problems over the past year. I now have many customers who do not want to use PayPal and want to pay directly with credit card. According to your plugin description, we can use multiple gateways including Stripe since 3.5 over a year ago. There is no reference to this option in your usage documentation. After all of the trouble I have had, I am not looking forward to adding more complexity with another gateway without first having a better understanding. In the settings, there is a selection to choose the primary gateway. What exactly does this mean? Is it simply displayed first? Please provide me with documentation regarding using more than one gateway.