That is what I have done....usually my long ride of the week on Saturday, and my long run on Sunday (with a spin in there as well depending on the week). Monday was off/rest. I always felt it was helpful if not just mentally to know I could do it. Plus when I was working, the weekend was always the best time to get long extended work outs in.

I also try to stay away from Sunday workouts so that I can attend church, although if Saturday is a rain out my wife usually lets me get in a make up ride on Sunday afternoon. The short answer is that whatever work/family/faith constraints you have... that is your reality and you have to schedule to match. Resting on the day of rest is great!

I followed the EN/Rich Strauss "Basic Week" when preping for IMFL but I never did any Sat/Sun back to back rides, although I took 3-4 half days off work to ride and hit it pretty hard on Saturdays. And while we are on the topic I think "The Book" coaching manifesto should be admitted to the canon of scripture as at least Deutro-canonical (like Tobias, or Maccabees ;-)

Sure, back to back rides MAY be beneficial (I'm not convinced), but they are by no means mandatory.

I have not worked with Rich or EN but I like his philosophy on training. From what I've read, he believes that intensity training is the key to their producing better than average results in their athletes. My guess is that the back to back workouts are to maximize your training within your time constraints. But breaking them up isn't going to cause you to fail so do what you need to do.

Have your written them though?

One thing I think they are trying to do is get you used to running on tired legs, after all if you are doing IM then your legs will be fatigued when you start the run. When I was with the Program with Pat M before it became EN I almost always had a long bike on saturday with a long run on sunday. Many times the plan notes would comment that the run was going to be tough due to the riding saturday. My coach now has me in a bike focus doing a longish (3hrs) ride saturday with 7 mile brick runs and then a straight long bike on sundays (5-6hrs). In a few weeks the saturdays will become straight bikes (3-4hrs) with sundays becoming bricks (6hr ride/4-5 mile run).
For me I really like the back to back big days because they get you used to running on tired legs so that come the big day when you hop off the bike and start out on the run you should be used to that feeling and know what to expect and where your pace should be for the run. Plus with monday being a rest day it really makes monday the best day of the week :)

Their philosophy now is long run on Thursday and long bike on Sat with a second shorter bike on Sunday with a 30 min brick run.
This way you are fresh for your key workouts. That is very important.
The short brick run is enough to see what it takes to get your running legs back without going long. This approach is much better for recovery as each workout works into the next.

I have only done the back to backs. Splitting the long days makes sense to maximize the efforts. For me, as a 1st time IM trainer last year, the back to back days (Iron weekends) were a great help physically and mentally. I gained a lot of confidence was able to start to get my mind around what I was going to do. None of my previous training had ever approached the level of doing a century ride followed by an hour run on day 1 and a 20 mile run followed by a 45 minute spin on day 2. If I decide to do another IM next year, I am going to follow the same plan.

Considering how my new plan has me cycling through non standard weeks I never get a weekend off, so while I used to hate Monday's now I loooove them.