Conceptually its pretty simple....
Change the name of CXG 8.14 from CrossOverGames.app to CrossOverGames814.app via the finder.
Install CrossOverGames 9.1
Then create the appropriatly named bottles (DDO814 and DDO91) from each application and install the game.
You WILL have to be careful not to launch the wrong CXG, and pick the wrong bottle. But after/other than that, it should be fine.
Note that you will wind up doing two complete installs of DDO, one in each bottle. (Oh yeah, and separate installs of PyLotRO in each bottle.) But, as long as you keep the naming convention separate and consistant, you should have no problems.
I use this technique with beta versions of both DDO and CXG clients.
... been working on LOTRO issues lately and haven't been doing DDO testing in the last couple of weeks. When last I tried, there WERE issues with the DDO5 update.