@Buttwoman and I were friends. She turned me on to double dongs, and many other anal toys. I have two double dongs. One is a purple jelly model 18" long and a little more than an inch in diameter. It could be the same one that @pres says below that she uses.
My largest double dong is The Python: https://www.extremerestraints.com/the-python-xl-double-dong.html
@Buttwoman walked into a 2017 Toronto Halloween party with a Python around her neck and its heads hanging down over her breasts. The python is 28 inches long. It's heads are 2.8 inches in diameter, and it's shaft is 2.43 inches in diameter.
Like @pres says below, the best way to prepare for a deep anal insertion is to use a long series of high volume enemas to empty your whole colon and get that empty, relaxed, anally horny feeling that makes your ass hunger for something huge.
Also like @pres and @Buttwoman, I enjoy inserting my 18" double dong all the way into me, letting my anus close over it, and keeping it inside me for extended periods.
The Python is another story. It's big. I have to slowly work one head through my anus. After the head pops into my rectum, it gets easier. I like to hold the opposite head stationary by squeezing it with my feet as I slowly move my hips back and forth to gradually shove more and more of my body onto The Python's shaft. I can swallow up 15 or 16 inches. That's my limit. The first time I did this, it triggered an instant orgasm.
The Python is designed for two people to share. I have a Python fantasy in which I find a partner (male or female) to swallow up one half of The Python while I swallow up the other. Then we blissfully bump butts (or crotches depending on how we arrange our legs), and fuck each other to orgasm.