On Tuesday, 8 October 2019 18:43:52 UTC+1, T i m wrote:
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?

Plugs are male, jacks are female.

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?

This is two male plugs and one female jack, I don't know if it's exactly what you require, but there are loads on Ebay/Google Shopping etc

https://www.ebay.co.uk/itm/splitter/333213876600

Two Male Ports: One for Headphone Jack on your PC/Laptop/phone, the other for Mic Jack on your PC/Laptop/phone.

Owain