At least for campaign play if not skirmish, you should be able to attach R2 to 3PO as a companion. Maybe for 1 additional threat on top of their combined cost.

What is the new companion rule?

Besbin Gambit introduces companions, but he probably doesn't know the companion rules yet.

(No errata is needed. It is certainly possible to introduce a companion-R2-D2 and/or a companion-C-3PO without touching the ally-R2-D2 and/or ally-C-3PO.)

Yeah, we don't really know exactly what all the companion rules are. All we know is that companions are being introduced (the Ugnaught's little astromech) but we don't know limitations and rules for the basics of companions and then how each companion manipulates or expounds on those rules.

If they errata away my ability to bring along the plucky astromech Droid and leave behind the annoying translator Droid, I'm gonna be pissed.

What is the new companion rule?

It's a secondary deployment card that attaches to another unit, allowing for groups consisting of more than one type of figure, with different stats and abilities. The companion takes its turn during the main card's activation, just like if they were a regular multi-figure group. The example we've seen is the Ugnaught Tinkerer with his Junk Droid, coming in The Bespin Gambit: https://www.fantasyflightgames.com/en/news/2016/3/4/the-bespin-gambit/

(No errata is needed. It is certainly possible to introduce a companion-R2-D2 and/or a companion-C-3PO without touching the ally-R2-D2 and/or ally-C-3PO.)

Not true. Either one of the existing deployment cards would need to be errata'd to say it can attach to the other as a companion, or, going with your idea for an entirly new companion-specific card, their mission would have to be errata'd to say it rewards you that card along with their individual cards. Either way some errata happens somewhere, and I think it would most likely be the way that doesn't involve FFG spending any money to print new cards unnecessarily.

I was not really talking about existing missions, just that an existing droid does not prevent the game from having them as companions as well. Companions will fill different needs than allies (or not?), and we have yet to see how useful they are.

Even if you want to use an ally gained as reward as a companion instead of a deployment card (usually you wouldn't, because you lose an activation in the process, but sometimes you want to have more actions per activation instead), you merely need an additional rule how to gain the use of a companion by gaining the corresponding ally, which does not need an errata to the mission.

You also don't need an errata for a hero sheet or a deployment card to be able to have a companion with a figure. The use of a companion can be gained by other methods than a figure-specific ability. There are a lot of card types added to the game by each expansion, all of which could grant a companion without changing the existing cards.

(So, no, I don't think they should be errataed, and I don't believe they would be errataed even if they were added as companions.)