I understand that they are fungible as far as the CME clearinghouse is concerned. What I'm more interested in, as SSB suggests, is how it will be implemented at IB - whether selling 1 SP or 5 ES on the floor will immediately flatten a +1 SP or +5 ES existing position in terms of buying power/perf bond requirements, account display, etc., flatten it overnight, or be carried as separate positions until you unwind them.
I have experience from a couple years ago with 6E and E7, where, even though they were theoretically fungible, both positions were carried in my account, and I had to liquidate each position separately instead of just letting them offset each other at final settlement.