Where can I find the regulation or policy clarifying if someone can attend back-to-back NCOES schools?

If that were policy you would find that in AR 350 - 1 or the atrrs course catalog prerequisites.
I've never heard of any policy like that. I attended SLC five months after graduating the career counselor course which counts as ALC in my MOS.
The problem with trying to attend a second ncoes is that you are the lowest priority on the oml to attend when you are not promotable

Talk to your unit training NCO or Officer. In nearly all things training, show me the regulation. If there is not one, no policy letter etc., then there is nothing preventing you from requesting the school.

Some schools have a policy in effect for an SM that fails the NCOES due to APFT or HT/WT that they cannot attend again for six months as well.

I went to ALC PH 2-3 back to back had one day between orders.

You can do this if you can get in, I am aware of no regulation preventing it, however your unit may have their own internal SOPs preventing it. And, even if there isn't an SOP against it, there are still several issues with this being a likely occurrence. One is that you cannot even enroll on ATRRS into one PME course without completing the previous one first, so you might get in too late to even get a reservation for the next course that soon. Further, you will be the bottom of any OML due to the fact that you couldn't possibly be promotable and just recently attended the previous course. Also consider this: If, for whatever reason, your unit/S-3/training NCO was able to get you in back to back despite all of this, you would still be taking a seat at that second course from someone who *should* be a higher priority than you.

It would most likely be in a tradoc reg not an army reg


https://adminpubs.tradoc.army.mil/index.html

I recommend you read DA PAM 600-25 and keep in mind going back to back for NCOES does not lineup with NCO merit based promotion, especially when your not eligible for promotion to the next rank. However, the new NCO merit based promotion is looking to improve NCO professional development by selecting fully-qualified (FQ) NCOs from the top of the OML for training early to qualify them for promotion (instead of after promotion selection). Therefore, if you fall into this category of leaders, you will be going to SLC in no time. Stay focus on the mission, take care of yours Soldiers, and do not neglect your personal and professional development, everything else will fall in place!