[Post editted with more information as it came available from comments]
According to the specs here the GFX Titan draws up to 250 Watt of power. It draw that via:
- The motherboard PCIe slot (up to 75 Watt)
- One 6 pin PEG (which allows up to 6¼ amp @ +12v)
- One 8 pin PEG (which allows up to 12½ amp @ +12v)
The sum of max supplied power: 75W + 75W + 150 W == 300 Watt.
The card uses up to 250W of the available 300 Watt.
Two Titan cards would draw up to 500 Watt. Most of that from +12v lines.
Now a typical power supply has several outputs which match different internal circuits. E.g. it provides +3.3v, +5v, +12v, -12v etc etc. Most of these are connected to the motherboard via a 24 pins connector.
In addition to that a typical motherboard has molex connectors with +5v and +12v options. (Some of them in the traditional molex form. some of them instead ending in a SATA powersupply connector which uses/provides the same +5v and +12v).
Lastly since graphics cards starting consuming large amounts of power (more than the mazx 75 Watt supplied via the PCI-e connector) we gained additional power connectors on the PSU. These supply only +12v, but at rather large amperages.
Given that rather large currents from though these extra connectors it seems desired to connect those without lots of extra contact points (read: as few conversion cables as possible. Any of these might have bad contacts and might add resistance. Large currents plus resistance is heat).
For this reason I previously adviced not to split/change the original cable set.
Up tow I just mentioned connectors and voltages. There is a reason for this. A computer PSU is not supposed to have overcurrent protection build in. E.g. on the yellow 12v lines it should not deliver more than 20 Ampere. Should something attemp to draw more power then a short circuit is assumed the the PSU shuts itself down.
This means max. 240 Watt (20 amp at +12v).
If we want to draw more power than that we either need:
- Multiple power supplies.
- Or a 'single' PSU box which contains multiple power supplies in one box.
There multiple powersupplies we call rails.
Thes rails can be properly build as real independend parts with their own control circuits and overcurrent protection, or they may be a mix where the PSU implements part of this. Regardless, for huge (>= 1125Watt) PSU something like this must be in use.
If there are trulue separate rails then you do not want to draw power from one rail to both graphics cards. (Remember, max 240 Watt, which plenty for about one PEG 6 and one PEG 8), but not for twice that.
So you want to check the PSU for rails and if possible connect one Titan to one rail and the other to a different rails. There rails are likely marked +12v-1, +12v-2 etc etc.
You wrote that your PSU has three PEG6's. Those are likely to be part of one rail.
Using thoese three (as PEG6 and [2xPEG6->PEG8 cable] should be OK. Leaving the other parts for supplying power to the other Titan using cables like these:
Note that a 1250 Watt PSU (which your 1125 is provided it has enough AC in) probably has 4 rails. But I still cannot find any good documentation on that.