So far with the packages Universal says you don't have to specify the date you plan to visit
Epic. If you bought a single day ticket my assumption (take that for what you will) is that it would only be good for that date you selected but that may not be how Universal handles 1 day tickets, guess we'll find how if APs have a different rule come Thursday.
The multi-day ticket would still fall under normal rules applying to the expiration date of the ticket such as if I go to buy a 3 day ticket say for May 22nd the validity date is
any 3 days between May 22nd and May 27th (I was also checking this up against the sales page for specifically including Epic in that multi-day ticket). Another example is a 5 day ticket starting May 22nd is
any 5 days from May 22nd thru May 29th. So this is just an assumption based on that but I would think so long as you used up your days on your multi-day ticket before the expiry date with the current rules of not having to use them consecutively you'd be okay to do what you proposed.
The issue people may run into is that expiry rule. A 5 day ticket for example includes 4 days at USF and IOA and 1 day at Epic leaving only 3 days of wiggle room (8 days total includes the 22nd, 5 days of park days) so putting in a 1-day ticket in between that doesn't alter your existing expiration date rules and leaves less flexibility for when you'd want to visit each of the parks.
This is the downer to the more restrictive expiration dates that Universal now has to use your ticket after the updated the terms several years back.