Policer does have a parameter called "index"; example:

===
tc filter add dev eth0 <..filterdesc> police index 30 

will share the bandwith with

tc filter add dev eth1 <..filterdesc2> police index 30 

the bandwidth is shared across devices (ingress or egress) and is based
on the index of the policer.
also the filterdesc is defining different flows, therefore you can share
bandwidth across flows.

I dont have any good documentation but maybe you could write it.
(The only related doc i have is future work described in:
http://www.cyberus.ca/~hadi/patches/action.DESCRIPTION)

I hope this helps.

cheers,
jamal