To enable dual boot on a T2 equipped Mac, one needs to change some Security settings first.

T2 Security Settings
BIAB 2021 Audiophile iMac Pro 4TB 128GB SSDx2 Catalina; 2012 MBP Catalina
Overture 5 DP 10 LogicProX Finale 26, Encore 5.0.7 SmartScoreX2P Notion 6
Legacy G4-1K DP OS 10.4.11/OS 9.2 Apps