So, once you get the girls panties off, you stuff them up her pussy with a bit left hanging out before you proceed with the single or double ass reaming. Just to emphasise that in good porn, that hole only serves secondary functions.
An alternative in the same vein would be to take a decently sized butt plug that starts off the scene in her ass and shove it up the other hole instead. Preferably one with wide base so that it 'plugs off' her pussy until it needs to be accessed for a dp. As I said, secondary functions.
What do you think?