Dual Bagpipes

Interesting to see this pop up here. I play a lot of traditional music, and one thing I'm more exposed to than most people is variations in bagpipes. The Highland Bagpipes of Scotland are a bit of an anomaly; Scotland obviously has a great piping tradition now, but historically there's not as much evidence of it as there is, for example, in England. However the English tradition died out almost entirely, with the exception of the Northumbrian tradition.

The Highland Pipes are also extremely loud and they're built for marching. They're not particularly designed for soft expression, as a lot of other pipes are. And honestly attaching two bags to the highland pipes would require such a ridiculous amount of pressure/air that you'd likely die from exhaustion :D

As mrob has already said, the pressure could vary the tuning, but it may be more possible to vary the sound using pressure when using a single-reeded instrument rather than the double-reed that most modern pipes use.

Probably the most amazing design of pipes is the Uilleann pipes, which have an insane range (3 octaves I think?), bellows for feeding the bag instead of a mouth pipe (not uncommon on other pipes as well), and something particularly interesting called the "regulators". These essentially let you play chords, or single notes, on the drones by using your wrist. This allows you to add accompaniment or rhythm into the playing. Here's an example of someone playing the regulators: https://www.youtube.com/watch?v=nZTHs7eQSKc

There are other interesting pipes that use multiple chanters, or combined chanter/drone systems that let you vary things more effectively. Some of my favourites are the Zampogna of Italy: https://www.youtube.com/watch?v=CU_xR_mS9YI, and particularly the odd Boha from Gascogne in France: https://www.youtube.com/watch?v=MKyXn2dqg9s. The Boha that the guy in that video makes are really interesting; he makes them with additional holes that you can move corks between to vary the scale, and he makes some with two chanters and a drone all together, which allow you to play simple harmonies. A technique which a guy I know has turned into a pretty phenomenal playing style (on some English-made pipes here): https://www.youtube.com/watch?v=fJisKPjCEXA

I assume it's not done because the tuning is affected by the pressure. Bagpipes typically play melodies over constantly sounding drones, and different sized pipes are affected differently by changes in air pressure, so bagpipes will sound out of tune if the air pressure varies too much (tuning errors are much more obvious in harmony than in monophonic melodies).

How would you couple the bags such that the pressure in them could vary independently?