Are these programs integrated with the event provider? Because if so you would need to be linking your Marketo program with a single event on the provider side, in which case I'm not sure how you'd manage events on two different dates in one program.
I'd personally split these into two separate programs for cleaner tracking. If you want to have a single form that allows people to choose a date, you can use some of the methods from Josh Hill's tutorial here:
http://www.marketingrockstarguides.com/setup-recurring-webinars-in-marketo-1479/