If Disney has about 15,000 resort guests and only half go to the EMH park that day, then you'll get an increase of about 7500 guests that day. In theory that could be a noticeable difference.
In reality I don't think it results in an increase that big. If 15,000 guests are staying in the resorts on a particular day and they did not have any EMH, we can assume the resort crowds would tend to spread out fairly evenly among the parks. (I know some parks have higher attendance than others, but statistically we'll assume an even distribution). This means about 4,000 guests per park. If the EMH park gets 7500 on an EMH day, then it only results in about 3500 more guests. I would think the parks could easily absorb that number without a problem.
Personally, I don't think it makes sense to park hop on EMH days. It takes about an hour to leave one park, wait for the bus/monorail or walk to car, travel to the second park, and enter the second park to begin touring. How much longer are the lines in the EMH park--10-15 minutes? You'd have to get on 4-6 more rides at the other park than you could at the EMH park to make a difference.
(Of course, park hopping does make sense if you want to take an afternoon break and go swimming, take a nap, eat lunch outside the park, etc. This scenario only applies to people leaving the EMH and going directly to another park because of the "higher" crowd levels at the EMH park.)
Unfortunately, Disney is very secretive with attendance info. We don't know average park attendance, capacity levels, number of guests taking advantage of EMH, and the amount of increase in attendance in the EMH park. So everything is speculative and anecdotal.