Happens a lot on heavy duty industrial engines. Usually, it's a low pressure turbo feeding a high pressure turbo.
Sometimes it's done for packaging, sometimes it's for efficiency, or one of several other reasons.
Saw this the other day, check it out.
https://www.carthrottle.com/post/vo...nology-uses-compressed-air-to-kill-turbo-lag/
Super stock diesels usually run triples (2 into 1) in the pulling classes.
Do a search for Haisley's and Scheids trucks, some good pics.
Have them on my 6.4. The theory is the small one spools really fast so you don't get a lot of turbo lag. Then the big one lights off and you make big steam.
Oh and the 6.4 Powerstrokes are compound setups, but they are a little weird. IIRC, the 6.4s have a small turbo feeding a large turbo, when traditionally it's the other way around.