"T i m" wrote in message
...
We are looking for a straight cable converter that would take a 3.5mm
4 pole jack that would typically connect a headset into a Playstation
or X-Box but split it out into a 3.5mm 'stereo' jack (headphones) and
a 2.5mm (mono?) jack for the mic?

It seems you can easily find them the other way round (two female
jacks into one male 4 pole plug) so I was wondering if there was a
technical reason why what we want to do isn't possible?

Nope, just not commonly needed is the only reason.

I think some PC mics were / are 'electret' (is it, 'powered') and so
require a 'stereo' plug (sig, power, gnd) so maybe that's something to
do with it?

Just something you need to allow for.