To use different spring versions with the same engine, there is pro workaround.
On gamestart the lobby will still try to connect to the game, but just close it during loadscreen. (I play windowed mode so can just click the "X", I think in fullscreen it should work to just exit as soon one is ingame)
Then just drag the script.txt from engine-A onto the spring.exe from engine-B.
For example like this: