Hi. What is the significance of two barrels in the 8900 and 8500? They supposedly keep better time towards the end of power reserve. Is there another advantage?
How do they work? When I wind, I'm simultaneously winding two mainsprings, or what?

 

Yes they supposedly make the movement more isochronous* but the downside, and it is a big one, is that the movement is made much bigger than a single spring alternative. Rolex can do 70 hours on a single spring, Omega can do 60 with 2 and 55 with one. Is it worth the chunkiness the resulting watches suffer from? IMO no.

*keeps time as well with a full and low power reserve.

 

Longer power reserve, less variation as the spring winds down are the two main advantages.

They both wind and unwind together.

 

As I've said before, watch movements are a series of compromises. For example one way Rolex achieved the longer reserve on the 3235 is to make the mainspring barrel walls very thin, so that more spring could be included in the same envelope dimensions. The walls are so thin that the barrel can't be used again, because there's no way to open and close it without causing damage, so the only option is to replace the entire unit.

They also reduced the fiction in the escapement by reducing the surface area contact between the escape wheel teeth and the pallet fork jewels. Assuming the same torque, this will concentrate wear on a smaller area of each of those parts, likely increasing the frequency of replacements required.

There's no free lunch...