Hi,
You have two options:
1) Register all of the phones to the same CME, and the 2851 would act as the master. Then you could configure the phones to fall back to the secondary CME.
2) If you have enough DSPs, you can register one set of DSPs to your 2851 like you have now, and also add a new 'sccp ccm group' for your 2811. Then you would have a set of DSPs registered to both.
There's not that requires that the DSPs used for a conference are on the same CME, like CUCM transcoding works.
The config would look something like this (overview)
dspfarm profile 1 transcoding
codec ...
associate application sccp
dspfarm profile 2 transcoding
codec ...
associate application sccp
sccp ccm x.x.x.x id 1 version 4.1
sccp ccm y.y.y.y id 2 version 4.1
sccp ccm group 1
associate ccm 1 pri 1
associate profile 1 register dsps_2851_cme1
sccp ccm group 2
associate ccm 2 pri 1
associate profile 2 register dsps_2851_cme2
Hope that helps.
-nick