Can I use the same version of c-treeRTG when using different versions of isCOBOL?

Yes. The only requirement is that the client library being used must match the version of the server.
For example, you may have isCOBOL 2021R1 and c-treeRTG 11.9 running on a machine and you want to also run isCOBOL 2022R1 with the same c-treeRTG.
Then you must use the ctree.dll on Windows or libctree.so on UNIX/Linux from the isCOBOL2021R1/bin or native/lib directory with the isCOBOL 2022R1 version.

For example, you can copy /isCOBOL_SDK2021R1/native/lib/libctree.so to your /isCOBOL_SDK2022R1/native/lib directory. Or in Windows copy the isCOBOL_SDK2021R1/bin/ctree.dll file to isCOBOL_SDK2022R1/bin.

Another way to look at this is that on UNIX or Linux, the libctree.so from the isCOBOL 2021R1 version must be in the LD_LIBRARY_PATH, LIBPATH, etc. when running isCOBOL 2022R1.
On Windows the ctree.dll from the isCOBOL 2021R1 version must be the first ctree.dll in the Windows search path when running isCOBOL 2022R1