While it's not advisable to have packages from multiple Debian release versions on your system at the same time, it is possible, and using release pinning can make this work mostly correctly.
Here's what I use to accomplish this in /etc/apt/preferences.d/pin-testing.pref:
Package: *
Pin: release a=stable
Pin-Priority: 1000
Package: *
Pin: release a=testing
Pin-Priority: 500
The generic format of a pin entry is
Package: name
Pin: (origin|version|release) ...
Pin-Priority: number
Release pins can be for v=10 (version number), n=bullseye (codename), or a=stable (archive name).
The apt_preferences(5) manual page has more info.