I understand that you place a rift progress marker based on matching the movement symbols on the tracks against what appears on the mythos card drawn. For example, the topmost track has a white moon & black plus sign, so if you draw a mythos with white moon & black plus, then add one rift progress marker to the top track (provided it's rift isn't open and the track isn't already full). But what about when you draw a double doom / next act card where all the monsters move? Strictly speaking, no single rift track matches the movement pattern on the card, which would mean no rift progress marker gets added. On the other hand, if you want to make it harder on the players (and isn't that the whole point?), one could subdivide the movement on the mythos cards into distinct subsets and thereby match three rift tracks (and thereby potentially add up to three progress markers).

I've played it both ways in recent games; when using the latter method it significantly increases the speed with which rifts open! My question is whether there has been an official ruling on what to do in cases like this?

Note that the official rules (p. 8 of the rulebook) would seem to favor the first approach, since they state "Each time a mythos card is resolved, check to see if the monster movement on it matches any of the patterns next to a closed rift. If it does, draw a rift progress marker ... ". However, since most double doom cards are nasty, it seems strange that they wouldn't have any corresponding activity with respect to rifts.

Officially, nothing happens with unopened rifts in such cases, although I like your idea!

Actually dj, when Kingsport came out, they ruled that if the movement doesn't match exactly, you don't add a doom marker. So those weird all monster move cards from some of the expansions cause no rift activity for adding rift tokens, they still cause the rifts to move if they have already opened however.