Hi there welcome to BW
A schedule is clock based and tends to have set feeding times and set nap times regardless of how baby is managing or adapting to that. A routine on the other hand is tailored to suit the individual baby rather than a one size fits all approach. With the EASY routine you may sometimes keep an eye on the clock especially if you have sleep or feeding issues to help you all settle into a regular routine however the routine is still based on you and your babies individual needs and is still flexible on a daily basis dependant on their needs that day.
The main aspects of EASY are to encourage good feeding and sleeping habits and to put a little A time between E and S so baby doesn't fall to sleep feeding (which can delay self settling skills and also effect how well they feed and sleep).
Tracy did say that low birth weight babies, prem or any other issues etc should be taken into account with regards to the average EASY routine. I don't know your twins but if they were born earlier than full term or had a lower birth weight you wouldn't be expected to clock watch for a 3hr feed time. You can still be flexible (just try to avoid snacking, LOs need to get the hind milk if BFing for instance so a good feed each time is more beneficial) as you work towards a more regular daily routine, maybe 2hrs or 2hr 30 mins for E? and still flexible 15 mins either way with that.
Any of the examples of timings given in the books or on the forum are only guides, Tracy didn't intend this to be a schedule to make a hungry baby wait an hour for a feed.
hope this helps