My initial thoughs were also at what altitude they passed each other. I imagine it would be mighty expensive to run a jet under 10,000 so I'd expect they were in at least class E which, as we all know, requires a transponder.
Perhaps another point to look at might be that the closing rate of these two higher performing craft would still have, more than likely prohibited either pilot from turning away even if thy did see each other
Some confusion there. This is the Macchi involved in the incident.
'See and avoid' isn't much help at a closing speed of around 270 + 120 = 390 kts shortly after descending through the cloud base
270 below 10? Naughty, naughty.