[Lenovo Zuk Z2 Pro ROMs, Kernels, Recoveries, & Oth] void23:
Ok, so I was playing around a bit more with DualBootPatcher, restarting from scratch, and now it's working quite well. I like the concept of being independent from TWRP, you just have to remember to patch _every_ zip accordingly before flashing. And that you cannot backup/restore individual ROMs in TWRP. My earlier "mistake" probably was that I had a normal installation of 2.5 stock rom, which was of course not tagged as primary. So well, in order to switch over to DualBooting you cannot just re-use an existing/running system it seems.

Here's the way I've now done it:

1. In any running system, install the DualPatcher app.

2. As already mentioned, use an older version of the app! (9.2.0.r237 works for me)

3. Pick your favourite ROM (2.5 stock zip for me) and patch it to primary with the app.

4. Install via TWRP just like every other ROM (wipe on 1st installation. Subsequent wipes only via the app!)

5. Reboot system from TWRP / configure ROM / ... / install DualPatcher app!

6. Pick you second/third/whatever ROM and patch it to data-slot-1/2/whatever (on Z2 pro we have massive space on data)

7. Pick Gapps/SuperSU/whatever additional zips you want to flash and also patch to data-slot-1/2/x

8. Install these data-slot-patched zips directly within the app. It complains the slot doesn't match the running (=primary) system, but that's just what we want, so continue. I have just flashed everything in one go (AEX 4.4 + OpenGapps Mini + Magisk).

9. In the ROM section of the app, the newly installed system should appear. Just click on it to set it current and reboot the device. It should boot the freshly installed data-slot-x system.

10. Configure new ROM / ... / install DualPatcher app! (In order to switch back to primary / install further ROMS / ...)

11. Voilà!

(Edit: Be aware that you cannot mix Android 7.x and Android 6 ROMs! Latest stock ZUI 2.5 is Android 7.0.)

Windows utilities are not needed, same for the TWRP-utilities. But it would probably be a good idea to have these around in case you have no working ROM or the app is causing trouble. TWRP-utilities 9.2.0.r242 has some issues on my phone with TWRP 3.1.0.0 though. It displays upside-down and touch isn't working. But you can navigate with vol-keys (blindly!) and confirm with power. Very cumbersome, but functionally working and able to switch ROMs etc. as a fallback.

A few words on App-data sharing: (Here is where DualPatcher really gets interesing!)

- It doesn't matter which ROM is currently running.

- Be sure to activate sharing (in DualPatcher app) directly after installing the app in question! (enabling sharing clears this app's data!)

- Reboot. Configure the app like you want it.

- Switch to other ROM where you want to access the shared data.

- Install app, enable sharing in DualPatcher, reboot.

- Open app and again, voilà!

I haven't tried it with that many apps yet, but it seems to be working fine in general. Issues may arise if the app internally manages its data with some ROM-specific stuff/whatever. For example, one app complained about "invalid tokens" for a saved password after switching to the other ROM. I suppose there's nothing to be done about that. But yes, in general it works fine and is in fact great!