Tcl: Shared directory for .tm modules across 8.6 and 9.0?

Hello,
I'm using .tm modules  on Linux (Debian) and currently have the following environment variables set (.profile, .bashrc, .xsessionrc ):
export TCL8_6_TM_PATH="$HOME/lib/tcl8.6/site-tcl"
export TCL9_0_TM_PATH="$HOME/lib/tcl9.0/site-tcl"
This works fine. However:
is there a way to define a shared directory for .tm modules that can be used by multiple Tcl versions, such as both 8.6 and 9.0?
Or is it strictly required to duplicate the .tm files in version-specific directories, since each tclsh only reads its own TCLx_y_TM_PATH?
Goal:
I'd like to maintain a single central module directory (e.g., ~/lib/tm/ or ~/lib/shared-tms/) for use across all versions of Tcl I have installed.
Best regards,
Greg