OS X is probably one of your best choices for this to be honest, as
the GEM devs have made rendering to DV via a YUV pipeline very very
fast.

The choice of intel and debian is mainly for cost. This is only one
exhibit of many, and I have just the one mac in my possession - a
portable, which I use for development.

Would you expect it would be a problem to run twin streams on a dual
head debian machine, and I really need to source another mac?

Can I get away without, so long as the intel box has some guts? (I have a spare 1.8 GHz single processor machine knocking around
which I had in mind, but maybe need something more powerful)

Cefn Curiosity Collective http://cefn.com/curiosity/