I have records only for when I actually pulled.  I've seen 2x/10x/summon or chase at least a few times, but not 2x/garanteed.  Not pulling for Goff on my main but thrilled to have him on my alt.

My question to others, is it odd that this champ and the last gatenteed ancient (Ragash) are both 100% aoe stun champs?

It is indeed very rare to have a 2x event on the same time as a legendary.  There's only 2 that I am aware of:

• Vizier was a guaranteed summon from 111 Ancients shards on the 10th November 2021 and a 2x Ancient event then occured on the 12th of November, which was 2 days into the 3 day guaranteed event.
• On the 24th June 2021 there was a guaranteed Ursala for 15 Void Shards, which was the same time that a 2x Void event started

A lot of guaranteed summons did happen straight after the 2x event, or straight before, but not overlapping.  The usual Plarium trick to encourage spending!

Also note that there has been about 25 guaranteed summons since they started, so 3 of them coinciding with a 2x event is about 10%