GDB extended-remote debugging and thread groups / inferiors

Hi

Does GDB support thread groups and/or inferiors with extended-remote 
debugging? I've read several texts related to that topic 
(https://sourceware.org/ml/gdb/2013-05/msg00082.html and follow-ups, 
https://sourceware.org/gdb/wiki/MultiTarget, 
https://sourceware.org/gdb/wiki/MultiProcess?highlight=%28OngoingWork%29 
and the official docs of course) - but I'm still very unsure whether 
what I'm trying to do should be working or not.

My situation:
  - GDB 7.6.50.20130604-cvs
  - My own GDBStub (supporting, among others: 
multiprocess+;qXfer:threads:read+;qXfer:osdata:read+;)
  - using 'target extended-remote' to connect
  - finally debugging a self-made multi-threaded MMU-less RTOS. All 
threads (kernel & user) run in the same address space (and therefore 
this system doesn't have 'real processes', just threads)

Recently, we introduced dual-core systems. When we run them in SMP mode 
we'd like to have 2 distinct GDB thread groups (because we finally we'd 
like to have our threads grouped by the CPU cores e.g. in Eclipse/CDT, 
because knowing which thread runs on which core is crucial in our case)

I tried several things:
  - Played with clone-inferior, add-inferior, etc.: I therefore extended 
my stub to return a process list with two 'virtual processes' with PID 1 
& 2, one for each core. But I don't see how I can use the *-inferior 
commands to attach to one of these "processes"
  - I tried -target-attach with no success either: This command caused 
the formerly debugged inferior to be killed before attaching to the second
  - I modified my stub so that it returns a thread list in which the 
threads of the different cores are part of a different PID (see the 
p1.xxx vs p2.xxx): $qXfer:threads:read:: => ...<thread id="p1.6a0b78" 
core="0">Idle</thread><thread id="p2.6a5188" 
core="1">Startup</thread>... without success: GDB crashes because it 
only creates one inferior (instead of two)

=> All these things make me think that I either misunderstood the 
purpose and/or usage of 'thread groups' and 'inferiors' or these things 
are still not properly supported for remote debugging. Can someone help 
me by giving some hints?

Thanks in advance,
Raphael