Just calculate the two rectangles individually,and see what common ,or near common modes they have.I use a free calculator that I downloaded from the Realtraps site.Having the L shaped room can be advatageous for a live room,just so long as you don't have to monitor from the unsymetrical end.
In the end,you may choose absorption for your bass trapping,no matter what stacked modes show up,you will need lots of it,and I suggest you fill every available corner.

What you wind up with is a coupling of the spaces. The modal response has little to do with this you can calculate it any way you prefer, but the echo chamber that the leg of the "L" produces supersedes the modal response.

for a Control Room

Thanks for the responses.

Knowing that it will produce excessive decay times; besides as much trapping in corners and the usual first reflections and rear wall, is there other known areas to look out for? Such as the diffraction around the corner of the 'L'. Should the 'extra' space be treated heavily to reduce the reverb chamber effect? What about the 'shadow' wall where the (LF) sound will bend around? surely it won't get as much direct sound, but am i wrong to visualize this wall could be treated to soak up alot of return? Or is there better use of resources?
This isn't my room, but of a pen-pal i'm trying to help. It would be so much easier if i could physically enter the room to measure the hot-spots and such, but that can't happen. Any more info welcome. Thanks again.