I disagree with this advice wholeheartedly. DO NOT do more than 1 away if you are average or above average on paper already. Just focus on killing that one away if you already have a home elective. I'm on the residency recruitment committee on my program and so many people end up shooting themselves in the foot with a less than stellar second away SLOE when their app would have been fine without that extra rotation. I mean, it ended up helping us, since it allowed us to realize red flags that weren't otherwise present in their application. But from an applicant's point of view, it's stupid to do more rotations than necessary since you risk exposing your weaknesses more.
I think that the number of aways someone should do is entirely dependent on what that applicant wants to do. I think we all agree that at least one away is required. But even for those applicants that are above average on paper, multiple aways can be helpful. Most folks are going to get decent SLOEs as long as they show up on time, are receptive to feedback, and continue to learn throughout their rotation. Those that receive poor SLOEs typically did something on their rotation to deserve them like saying something inappropriate, showing up late, being disinterested, or demonstrating poor progression of learning.
One thing we don't talk about enough are the benefits of multiple aways. Many students are only exposed to one type of hospital during medical school, be it academic, community, or county. Each of these types of programs differ greatly in how lectures are given, the types of patients you see, and the resources at hand. It is also extremely hard to get the flavor of an ER on an interview day. You don't get to see how support staff interact with the residents, what type of scut work occurs, how consultants treat consults, and what type of patient population you can see. The interview day also doesn't let you know what type of on shift pace and learning environment you prefer, how much autonomy is afforded to the residents, and how much cleanup there is after a shift. Rotating multiple places can give you all of this and more. Speaking from experience, I did 3 aways plus my home institution (2 academic, 1 county, 1 community). Prior to going on my last away, I thought it was going to be my number 1 on my ROL given it was back home, a very well respected program, and a fit for my SO. Thank god I rotated there because I did not enjoy my time in the department, but loved the residents and the staff. I doubt I'd have known this if I hadn't rotated there.
For most applicants there is not a one right answer. Yes you can match with a home rotation and an away. You can also match with a home rotation and 3 aways. If your evals on your 3rd year rotations are generally good and don't say that you are a creeper, your SLOEs should also be fine. It is true a bad SLOE can torpedo your applications, but if you have an outlier SLOE with 2 decent ones, most residency committees will ask you about the SLOE and realize that it was more of a fit issue.