I'm saying, Madara and Izuna didn't do it because they didn't know about EMS. Madara was half blind, and so he took his brothers eyes. It was pure coincidence, IMO, that he obtained the EMS. I believe it was sheer luck that Madara managed to find out how to unlock the EMS. So what i'm saying is, it could be possible to simply switch eyes in order to gain EMS (no-one has tried it so we cannot know) and it was simply coincidence that out of the two pairs of people who could've achieved this dojutsu through this means (straight eye swap), two of them were blind aswell (Itachi and Madara), so even if the 4 brothers swapped eyes, we still would not get a valid answer.
1) Are you SURE that they didnt know about EMS? Yes, it was stated that MS was first unlocked by Madara and Izuna. But once the entire clan saw this, I'm sure it prompted alot of Uchiha Shinobi to try this until it was replicated by many shinobi. Which means after Madara/Izuna got it other Uchiha learned the secret and started attempting to do the same. Which brings me to my point #2.
2) Secondly, I'm sure that the Uchiha Stone Tablet was MORE DECIPERABLE after Madara recieved the MS eyes. It would only make sense that the creator of the stone showed you how to achieve the "next eye level" by the one you already achieved. In other words, having Sharigan eyes reading the Tablet would reveal how to get the MS, having MS eyes reading the tablet shows you how to get EMS, and EMS eyes reading the tablet shows you how to get the Rinnegan. Itachi learned from the monument how to get MS, how else can this be explained...?
3) Lastly, by Itachi's last words in the last manga, the Uchiha had a habit of Spaming Kinjutsus and taking more eyes after they were sealed. I'm sure eventually someone else may have been able to accomplish this feat that has not been mentioned as of yet. This is because of the practice of taking eachothers eyes that came from the over useage of Izanagi. FYI
-gHost Senju, K.U.S.H. Sage