OK no problem,
might see if I can get some of my colleagues interested in taking a look into it.
On the HVR-4000, no, you can't use both frontends at the same time. You can use one or the other.
MythTV handles this idea by allowing you to assign frontends to "groups", and it will only attempt to use one frontend from the same group at a time.
ls -lR on the /dev/dvb directory with the HVR-4000 adapter shows you this:
crw-rw---- 1 root video 212, 4 2011-03-08 21:47 demux0
crw-rw---- 1 root video 212, 8 2011-03-08 21:47 demux1
crw-rw---- 1 root video 212, 5 2011-03-08 21:47 dvr0
crw-rw---- 1 root video 212, 9 2011-03-08 21:47 dvr1
crw-rw---- 1 root video 212, 3 2011-03-08 21:47 frontend0
crw-rw---- 1 root video 212, 7 2011-03-08 21:47 frontend1
crw-rw---- 1 root video 212, 6 2011-03-08 21:47 net0
crw-rw---- 1 root video 212, 10 2011-03-08 21:47 net1
regards
Liam