I did a back-to-back at ARN (Stockholm) yesterday. It worked, with a few caveats and some differences from previous descriptions.
We arrived at gate 20, and I couldn't see any signs for higher-numbered gates such as "70, 82-87". From the gate, the route was up one level and along a corridor. Signs there pointed ahead to arrivals. Signs for transfers were present but had a previous arrow in the other direction covered over, and there appeared to be no way to go in the formerly-signed direction. I suspect BA is now arriving in a different part of the airport and the airport has been rearranged a bit.
I went forward to the immigration line (passport check), which has "EU/EEA" and "other" lines. Immediately after this, there is a left turn to "transfer" (and a right turn to arrivals). Turning left, there were staff at the transfer security and they had zero people waiting for them. Security screening was rapid and efficient. The staff helpfully asked if I knew which gate to go to, and directed me appropriately (while I know we arrived at gate 20, that wasn't obvious on arrival, only on departure).
Then downstairs to a shopping and gate area, forward towards gate 20, through the immigration passport check, and I was back at gate 20. There were no queues at the immigration exit line.
From arriving airbridge to in front of the departing gate took me about 12 minutes, but there are caveats:
- I hold an EU passport. I have no idea how fast the non-EU line was going. You would obviously need to be able to enter the Schengen area to use this path, whether with EU or non EU passport.
- I was among the first few people off the aircraft, and the queue at the passport checks was short in both cases. Had they been very long, I would have been in trouble.
- I think only a few gates feed into this passport line, but if any of these had a flight with a lot of third-country nationals and you were behind them the queue could be very slow indeed.
I also had Verifly problems - I could not work out how to get myself verified so I could do OLCI for the return flight (onwards to the USA). I'm aware there's an extensive thread on Verifly, and with much more time this might be workable, or it might not. I also couldn't OLCI for the outbound as my documents required checking. Getting a boarding pass for the outbound at Heathrow was easy, but getting the return boarding pass required trying a couple of agents before I got one who would just check me in for the first flight.
As an aside, the crew were very curious to see me again ("That was a rapid turnaround!") and asked me about it later in the flight when I was talking to them. The senior cabin crew member did guess, correctly, both 'cheaper' and 'gets more [tier] points'.
Overall, this was (particularly with respect to the boarding passes) more fraught than in the times before the pandemic, but still do-able with significant caveats regarding the passport checks.