Hi

As things stand at present you cannot split the services of a single FBV foot controller  between two devices directly in any supported way.  It has been done using an FBV Shortboard MkI and a Flextone 3 amp plus a Spider Valve MkI by another Line 6 Expert User (spaceatl) and it worked but there's no guarantee that that method would work with two POD HD Pro units, plus if you attempted it and damage was to occur as a result it would not be supported by Line 6 and you would void your warranty.

If one device were able to act as Master and the others as slaves via MIDI then it would be possible.  With the POD HD Pro and the POD HD500, at the moment with the current firmware, the HD500 and the HD Pro can send MIDI Program Change, Bank Change and CC messages BUT currently incoming MIDI is restricted to only Program Change and Bank Change commands.

I hope that Line 6 are going to increase the MIDI capabilities so that two or more units can be controlled from one FBV connected to a master unit, but there has been no announcement so far that this will happen, so I am speculating.

For now, depending on whether you want to run the HD Pro units in parallel or whether you want to run them in series, you could possibly split things up so that things you need to control such as certain FX (on/off/expression etc...) could be in the first (master) unit and other settings such as amp models which might need less direct control could be held in the slave unit in a series configuration, so effectively you'd have a pair of presets, one in each unit, to make up one combined or 'big' preset.  The 'slave unit would simply change presets when you select a new preset on the master.  You could turn FX and amp models on or off on the master unit and you could apply expression changes using the expression pedal on the master unit.  This could still be quite flexible although I realise not as flexible by a long way as full MIDI implementation would allow.

I can see a lot of benefit in being able to hook up multiple units in the way that I think you envisage, but right now you are somewhat limited in what you can do.

Nick