How can I connect multiple lights to a two prong timer?
Two-prong splitters are not hard to find. Expand your search to big box stores or online retailers.

You can also use the ubiquitous 3-to-2 adapter (cheater plug):



This adapter is safe to use in this case because of the GFCI outlet.

However, if you're in a pinch, just break off the ground prong on that green splitter and plug the ground holes on the outlets with caulk or something else non-conductive.

Don't use it for anything requiring equipment grounding. You're responsible for acting like an adult and not abusing this equipment (or a cheater plug, or whatever) or allowing less clever people to do so as a result.