Need advice on advance fisting
I need some advice on fisting and stretching.
I have been practicing for about 7 years now, been able to get fisted for 3-4 years, it's been amazing but now I'm trying to push my limits a bit and go for double fisting. I took training a bit more seriously and bought some new toys. Before that, i mostly used plugs and dildos of 4-5cm (diameter) and occasionally a 6cm plug (i can't take a 6cm dildo more than 15-17cm without the usual block of the end of the rectum going into the colon).
Since i wanted to train without my partner too, i bought 3 plugs, 6.5, 6.8 and 7.5cm. It took me a few weeks to get accustomed to the 6.5 and the 6.8 which are both PVC, but the 7.5cm is glass and it took me another few weeks to just manage to take it entirely. The issue is, when i finally managed to put it fully (i managed before to put the biggest part at the sphincter, but never dared to put it fully in, as i was worried I wasn't dilated enough to push it out without pain), the thing that was mostly blocking me from pushing it out was not my sphincter, but my bones (the ones on the sides, not my coccyx). I tried a few times and every time, whatever the position, i felt like this was mostly what was stopping me from taking it easily.
I don't think that fisting would be that much impacted because hands are way easier to shape in order to fit properly, but what position would you recommend to avoid feeling this/having this block me?
Thank you for your answer, and obviously pic related