This mod works but it's not safe

First, some precautions:
- For anyone who want to connect 240V to another psu, please make sure you're using a correct rating (volt and ampere). You can't cut yellow (+12V) and black wire (0v) from old psu and use it with 240V. What will happen is, you'll get zap even if you're only touching the insulator rubber/plastic/etc.
- Having 240V at 50Hz isn't very friendly with pc due to EMI or RF interference. Try to "route" it far away from any internal circuitry/wiring/board. Or, use shielded cable (which is expensive).

Why it's not safe :
- You increase the chance of something goes wrong. 1 psu might have a certain chance to blow up, 2 psu will likely double it.
- There's no protection from reverse voltage (the current flow backward, coming from other psu inside another psu). Happen when one of the psu faulty (ie: psu output 0v instead 12V, which means the current flow from 12v healthy psu down to 0v in this faulty psu. Component inside faulty psu now stressed with -12v. Or, faulty psu output is 14v instead 12v, now the current will flow backward to the healthy psu. Component in healthy psu now stressed with 12-14=-2v)

What you can expect when psu is fault:
With single psu, you'll only get "poof" once. With double psu without protection, you'll probably get "poof bzzt poof". So, double the psu, double the explosion
The good thing is, you'll need to replace everything anyway, so it doesn't matter how many time its exploded.

The real solution:
- Google diode-or or FET-or application note.
- Problem with diode-or or FET-or: voltage drop across the diode/FET (eg: instead of getting 3.3v you'll get 3.0v, 0.3v loss in diode) .

tl;dr:
It works but there's a few caveat.
One can get away without diode-or if he's only using it to power isolated things like fan (with no rpm feedback, so there's no connection from psu to another psu except the 0v.)
Others like vga,hd, etc isn't isolated (voltage level/signal for data is sourced from that psu, so it'll connected to mobo and to another psu)