I have 2 B2150's. Located an old filter. Just one cylindrical body with plate on one end for wing nut attachment to the metal cone that serves as air intake. I have a "Workshop Manual" for the B1550, 1750, 2150 series. Only one small picture there with a note on the air filter and it shows a single stage filter. It does ring a bell that there may be a dual inner and outer filter version. It's not like one feeds the other through tubing or something. If you have two, there would likely be an inner cylinder and an outer cylinder of filter material covered with steel mesh. Just take the filter housing apart as you normally would for inspection/replacement and see what you have.
Yes. I expect you'll have to go to a dealer for that filter. Not all that common and I doubt you could find it in a parts store, though these days you can find darn near anything on the web.
Most of the newer stuff is designed and made by Donaldson. Not sure about yours but my L3940 inner and outer air are identical to the Exmark zero turn mower I just bought but half the price from Exmark or Kohler.
AMERICAN PARTS 94496 BALDWIN PA3476 BIG A 94496 BOBCAT-MELROE 6682494 CARQUEST 88496 CASE-IH CNH GLOBAL 200045A COOPERS AEM2634 FLEETGUARD AF4939 FLEETRITE AFR84939 FRAM CA7088