Unfortunately no.

The Geany keybinding dialog has a fixed list of actions that can each only be assigned one keycode.

The dialog has a number of issues as well as this, but no one has attempted to recode it. So it is unlikely that it will be changed.

I'm not sure, but it seems you can try a workaround with the GeanyLua plugin (see docs/geanylua-keys.html). Something like SwitchToRightDocument.lua (with your example):

geany.keycmd("NOTEBOOK_SWITCHTABRIGHT")

@elextr
Ahh that is unfortunate, thank you.
You mentioned that the Preferences dialog window has a number of issues.
Probably better that I do not know what they are :) ..... but
Are they more coding based issues that only a developer would understand or are there any more layout / feature request based kind of issues?
If they are layout/feature based let me know and maybe I could at least type out some posts for the Ideas section just in case future developers are interested in making the updates.
Up to you.
I will make a request post in Idea section for this just in case.

Thank You

@Skif-off
Sorry I did not realize you responded, thank you for this.
Also took me a little while to be able to test this, even though I knew of the GeanyLua plugin and its use (because a different plugin I was looking into required it) because Geany is used for creating scripts, I still was viewing that plugin as if it added help/tools for scripting in Lua, rather than what it actually does, providing Lua scripts to be used by Geany itself. Hopefully that makes sense :)
Anyways, good idea I was able to try this and it works great.
As mentioned above, I think I am still going to put in a request for this, but as @elextr mentioned this probably is not going to be added, so thank you for providing this solution.
Can I just ask you one question, what you provided is the main keybinding I was looking for at the moment....
geany.keycmd("NOTEBOOK_SWITCHTABLEFT")
geany.keycmd("NOTEBOOK_SWITCHTABRIGHT")
but where did you find the "NOTEBOOK_SWITCHTABRIGHT" actions name?
Do you know of a list of actions somewhere, because I might want to assign other actions to more than one keybinding.

Thank You again

where did you find the "NOTEBOOK_SWITCHTABRIGHT" actions name?

In the plugin's source code, glspi_keycmd.h is mentioned in the description of the plugin's functions (see docs/geanylua-ref.html).

@Skif-off
Ahh I am glad I asked :)
Thank You.