I would think that because women are all built so differently, it would be almost impossible to come up with a universal toy meant for that kind of action to fit correctly.
Also another thing to consider here is that, having a toy that goes in both the vagina and rectum, and is also a dong, for lack of better terms, having one that doesn't fit properly might do some damage to our thin wall that separates the vagina from our rectum. I do however think now that you mentioned it that maybe putting in a butt plug as a separate unit while wearing the feeldoe might make things a little tighter in there, which would in turn keep it up in place a bit better.
As I said in an earlier post, I do use a harness with our feeldoe because I don't want to have to give up a hand to keep making sure it's staying in place. However, due to the way the feeldoe sits, I had to be choosy about the harness I bought because a lot of them have the hole for the dildo way too high on the womans pubic bone which would still work, but would cause even more pressure in the wrong area to make the thing want to pop out of place.
Also, I have to have as much skin to skin contact as possible. So the harness I bought is tiny. It has a little triangle base in which the feeldoe sits and comes out of and a few straps, sort of like in a g-string type of fashion. If I had my choice, I wish I didn't have to use it, but the fact is, if you can get it inside before you start having sex, by the time you start ramping up, our juices get flowing and flowing making it almost next to impossible to keep the thing up inside and placed correctly.
I will be honestly with you, I have a lot of trouble getting it inside me with lube on it. After it kept working it's way out, the next time I decided to insert it without any lube at all. Well, that took some doing, okay, a lot of doing, and a little pain, thought that might keep it up there since there woldn't be the extra lube as well. Well, doing that to Doc excites me, so I provided enough lube of my own after a few minutes and the same thing happened.
Eventually someone will figure it all out. But for now, we have to work with what we have, and hope and pray that the toy makers get enough feedback from users like us to see and hopefully eventually be able to fix the issues with the products.
Mashie