B!
I used to think like most of the above posters, and then we tried a split stay. Now I love the idea. Of course, it does depend a little on your style and the time of year.
During one of the really crowded times, it pays to be able to get out early, and stay out late - so having a moving day can mean losing a park day....but that's hardly the end of the world. It forced us to stop and see some of the 'extras.' Especially when our second hotel was AKL!
Even during a non-peak time, it will cost you some time...but it is totally worth it - especially if you plan to take advantage of what each resort has to offer. Like on your moving day, take advantage of the swimming at BC, walking the Boardwalk, and the free boat trips at WL.
Since you're thinking about two deluxe resorts - I'd totally take advantage of the location of each one. Plan your stay to take advantage of EMH at MK, and your BC stay to take advantage of EMH at Epcot and MGM (plus various meals near each location.) I think if you do that, you'll really get a bang out of the two locations.
We often stay at the Swan. Recently, we decided to stay at POR. It was nice, but I hadn't really though out how much harder it would be to hop into the WS for dinner (after an afternoon break) if you aren't staying in that area - and especially when you have a slow walker in your group. If you're in that area though, it's really easy to take a break from Epcot - swim, then get gussied up for a fancy dinner at one of the fancy WS restaurants. So I think staying at the BC will really be a treat in that regard.
If you do decide on a split stay, I suggest that you strategize your (un)packing a little too. Maybe pack the clothes for the first part of the trip together, or put everyone's dirty clothes in one suitcase - and (re)pack all the items you'll need for the second part of the trip together - remaining clean clothes, toiletries, etc. It also helped us to put kids' outfits (shirt, shorts, socks, etc.) in ziploc bags when we did our original packing. the last few days, we only had to use one suitcase. It actually made our final checkout much easier than having to pack up a week's worth of stuff. th eonly thing left to pack on our last day was the toiletries and a few toys!