I haven't tried as I don't use the legacy Cumulus, only CumulusMX, but in principle it should be possible to run two Cumulus instances on the same host/server (old + new or even new + new).
I think you have to install CMX into a directory named \CumulsMX (or ../CumulusMX if Linux based) somewhere in your file system - and not in contact with your still used legacy Cumulus directory.
The CMX admin interface runs by default on http port 8998 (and possibly the old Cumulus too, here I'm speculating).
If you start your CMX from inside the ..\CumulusMX directory in a console window and the command line (Windows)
...\CumulusMX> CumulusMX.exe port 8999 it should come up and can be accessed via
http://IP-CMX-host:8999 or
http://localhost:8999
(under Linux and MacOS it's in principle the same, only you have to start CMX via the mono library (CMX v.3 - e.g. sudo mono CumulusMX.exe) or the dotnet library (v.4).)
like this they would run independent of each other.
When you use a GW1200 with CMX (GW1202 is a station model name, package, kit), you configure your station in CMX as "Ecowitt local API" and provide the IP address of your gateway. (Can be seen in the WS View Plus app # or in your router).