Double penetration can be difficult to do if the angles or the timings aren’t right. You especially have to have two fully-grown people in one place so it can be difficult to find the right positions to get into and getting the rhythm right too.
One party may be trying to penetrate in and out and the other may be doing another speed. The woman on top may be trying to go up and down or side to side or back and forth. It takes a lot of coordination to “get it right”.
Men can often feel each other’s penises through the thin tissue separating the vagina and the rectum which can make some heterosexual men feel a bit weird about. It also requires a lot of lube, a bit of creative thinking and some coordinated timings. The best way I can describe it is being very “filled-up”. It’s quite overwhelming, but feels pretty good.