Oh man, these conversations quickly degrade into "I believe", "I think" and "The guy down the street / on some random forum says...". It is hard to argue with belief when silly facts get in the way.
As with many things, whether or not you can "get away with" using an 8 pin splitter .. "It depends".
I used an 8 pin y splitter before I bought the proper supply to run a pair of L5640's. Those are 60 watt TDP processors. And, we all know how reliable it is to use TDP for your power budgets. I think I bought mine on Amazon. They work fine within their limits.
One thing to consider is the power supply. Is it single rail or multi rail. If it is multi-rail, what is the limit on the rail feeding the ATX12v/EPS12v plug? Is it the UL 240VA limit? Many multi-rail supplies are limited to 18amps at 12v per rail to comply with the 240 VA UL limit. If so, you probably won't melt the plug going into your splitter with a pair of 135 watt TDP processors before the over current limit kicks in. (The UL limit is specified in VA, and VA == watts for direct current so we can use them interchangeably in this discussion. This is not necessarily true for AC. But, I digress). But, you won't be able to run a pair of 135 watt processors flat out before the over current limit in the power supply shuts you down.
If you have a single rail supply, or the rail feeding the ATX12v/EPS12v plug is NOT limited to the UL 240VA limit, you have to ask yourself, how lucky do you feel? Do you think the pins in that 8 pin connector are good for 5 amps? 6? 8? A conservative number is 5 amps, or 60 watts per pin, or 240 watts per 8 pin connector. How about 6 amps per? Four pins at 6 amps will give you 24 amps or 288 watts which will run your pair of 135 watt processors. Allowing for some variation in plug resistance, you may have some pins pulling 8 amps and others only pulling 4 amps but even 8 amps is unlikely to melt the 8 pin connector feeding the splitter.
So, if you have a single rail supply rated, or if you have a multi-rail supply, and that multi-rail supply can provide 25 amps on the rail feeding the ATX12V/EPS12V plug, then you can probably get away with splitting the single ATX12V plug into two ATX12V/EPS12V plugs to run your 2670's.
A safer choice is to use one of the dual 6 pin PCIe to single ATX12V adapters if you have spare PCIe power plugs. They are likely on a separate rail for a multi-rail supply. They will be using separate wires than the 8 pin ATX12V plug so you are less likely to overload the wires or the pins. They are harder to find than the 8 pin Y splitters.
And, of course, the best and safest way is to use a supply with dual 8 pin ATX12V/EPS12V plugs.
You might get away with using the splitter. And, you might burn down your house by using one. The former is far more likely than the latter. Please don't construe this as legal advise and don't sue me if you do burn down your house using an 8 pin Y splitter.