DualBootPatcher is an open-source app that allows multiple ROMs to be installed on a single Android device. It does its best to work with existing code and does not require explicit support from ROMs. There are currently more than 250 supported devices and their variations. The list of supported devices can be found here.
Website: link XDA thread: link
Downloads can be found here.
See the docs/build/ directory for instructions on building for Linux, Windows, and Android.
The patcher is licensed under GPLv3+ (see the LICENSE file). Third party libraries and programs are used under their respective licenses. Copies of these licenses are in the licenses/ directory of this repository. Patches and other source code modifications to third party software are under the same license as the original software.