Double penetrations can be done in all scenes, but require a little bit of setup, depending on if you're a dominant or submissive in the encounter:
- As a Dominant, the other person has to be allowed to perform penetrations on their own. Other dominants will be able to do so, but other submissives will need to be given permission by you, which will require you to switch your action choices to them and then select "Allow penetrations".
- As a Submissive, you should* be able to manually initiate DPs when you're already being penetrated and the other dominant is in position to penetrate you.
The big thing, though, is positioning. The most reliable position from what I'e found is to have the penetrated in Doggystyle, then have one character set to "Behind" and the other to "Humping".
As for editing permanent NPCs, not with in-game mechanics (barring the couple that you can enslave and thus transform however you want), but since saves are uncompressed text in this game, you can open your save file in a text editor and change their parts. There's also
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, another tool, to streamline this.