Gazing_south, do you mean streaming a recording from one Humax to the other, or copying (via network or external media), then playing?
I don't have a Humax, but from the specifications on the IceTV Store for the 7500, it looks like it can be a DNLA client, but not a server (so streaming from one to the other using DLNA would not be possible) (Incorrect. See Dave at IceTV's post below).
Beyonwiz PVRs can stream recordings between themselves (proprietary protocol, though), but don't support DLNA. They can play back media over the network from Windows shares (e.g. on PC or NAS). Unfortunately, they can't play back from Windows shares hosted on Mac OS X Lion (10.7.x) of Mountain Lion (10.8.x) without replacing the Apple SMB server with a free third-party SMB server. A downside is that only the most expensive Beyonwiz model (DP-P2) and the least capable model (DP-H1, single tuner, record only to user-supplied USB HDD) appear to be being sold new now. Other models appear to only be available refurbished.
[Beyonwiz beta tester]