@TKitten
You aren't doing anything wrong. Let's take a look (figuratively, of course).
First, the warning about not overinflating is a valid one for any of the double balloon nozzles, especially those with a fluted design. Once the balloon is inflated so that the fluting disappears at the point of maximum diameter, extra squeezes of the inflator bulb do not significantly inflate the enlarge the balloon. Beyond a certain point the pressure begins to break the bonds in the elastomer. Latex rubber has more tolerance than silicone rubber balloons for this.
Second, you are getting a lesson in how elastic the anal sphincter muscle tissue really is. If it wasn't, the tissue would have torn (anal fissure) and you would like have been bleeding. Small fissures usually repair themselves. Full blown tears require surgery. As mentioned, it could also be rectal prolapse which is when the lower part of the rectum is pushed out through the anus.
There is a misperception that the inner balloon should seal the inside surface of the anal sphincter ring and the outer balloon should seal the outer surface of the anal sphincter ring and it should all be held in place because the anus is "too small" and inflexible to allow anything to pass outward. Hence, the idea that the two balloons are spaced too far apart. I invite you to read and look at the pictures of the Sandan XL in my blog and album (https://en.zity.biz/index.php?mx=blog;ox=showpage;pg=7361 and the images https://en.zity.biz/index.php?mx=gallery;ox=showalbum;ax=12195). If I inflate the front (inner) balloon and the outer balloon, it acts and seals as well as any double balloon nozzle. It's not as flexible as a standard double balloon nozzle but you can see without the middle balloon inflated, there's more than an inch spacing between the inner and outer balloon.
I can push it out if I move my hips to a certain position (which might be a factor in your pushing the inflated balloon out).
What I've found is that it's the friction/engagement of the rectal muscle walls with the balloon surface that holds the balloon in place (do you use lubricant to insert the nozzle? You may be using too much. You only need enough to limit friction when it's passing through the anus).
I've written about this before. Once the nozzle is inserted, it takes more than just inflating it. I use a sequence of Kegel contractions with the inflation of the inner balloon like this: squeeze of the inflation bulb, 7-10 Kegel contractions, next squeeze, 7-10 Kegel contractions, until I've reached the 6th squeeze of the inflation bulb. Don't be in a rush to get it inflated. You should start feeling a significant fullness at the 3rd to 4th squeeze of the bulb.
Give your rectum and rectal muscles a chance to adapt to this enlarging object.
As you do the Kegel contractions, it should feel like you are drawing the nozzle into you (you are in a way). This helps the rectal tissue engage with the balloon surface as you inflate it. You should be able to tug on the nozzle slightly once it is inflated and tell that it does not move and that it has effectively plugged the lower end of your rectum. The outer balloon inflation is the true test, however. When you inflate it (Kegels not required), if it feels tight against the outer surface (it's quite a wickedly delicious sensation IMHO), you are effectively sealed.
As mentioned, too much lubricant can make it difficult to maintain the friction between the balloon and the rectal tissue. The same thing can happen with soap. If soapy liquid can work it's way around the balloon surface, the friction can be greatly reduced.
I hope this helps. Once I started doing balloon inflations this way, I had fewer issues and the various balloon nozzles became much "pleasurable" to use.