I'm late to the party, and tons of great advice in this thread about how to proceed to install a second EVSE, but my number one question for folks in this situation is this: do you actually need two charging stations?
We've been a two EV family since 2013 and for most of that time have basically been able to get by with a single charging station. The one time it didn't really work for us was when we had a Chevy Volt and a 2012 Nissan LEAF, both which only had 3.3kW chargers and thus took hours to charge (and both needed to be charged daily). For this, I built a J1772 hydra splitter that could split my 7kW EVSE output into two connectors, but that's a different story.
Now that we have two Teslas, each of those only needs to be charged every few days, so it is a trivial matter to just alternate which vehicle gets the charge each night.
Unless you know you have excessive driving needs, or there are physical reasons for having two EVSEs (cords won't reach), I would at least see if you can manage with a single EVSE before plunking down funds on a second one.