To start off with, welcome to the site. There are a lot of good people here with a lot of good info and help.
As to your question, the only two that I've had anything to do with are the;
- Savage model 24 and some of its variations and the Brno ZH304. In the Savage, the closest thing I see to what you've asked about is the 30-30/12ga and the 30-30/20ga. I believe there is also a 308/12ga variation.
- Brno ZH304 I have is in 7x57R/12ga. In the 300 series there are a number of other calibers available and some with a 16ga instead of 12ga but I don't think the 20ga option you would like is available.
There may, and likely are other manufacturers of combination guns along this line but Brno & Savage are the only ones I've had anything to do with. Oh, the rifle barrel is on top.