Adding a second lithium battery

[alanerickson]

I have a recently built 19' and it came with a single lithium battery and solar panel and the GO Power solar controller. The Go Power controller manual indicated that a second battery can be connected to the controller. If I add a second battery should I add it to the controller, or is adding the second battery in parallel with the first battery sufficient? I assume adding to the controller is preferred, but wanted to double check before going through the extra work that this will require. Thanks.