It depends on the power supply, Just look for a SLI certified PSU. Some power supplys, or most of them have two twelve volt power rails as standard. some have one with a high amp rating on the one 12v power rail. What you should do is check how much power, in watts each of the 7870 cards draw from the PSU, and also what each card requires from each of the 12 power rails in amps for each card. work it out and you will get some idea of the wattage of power supply you will need. checking what each can output in amps on the 12v power rails as said. Stick to a quality brand and don`t be tempted to go for a cheap one at £50 or so. Ocz, Silver Stone , or a corsair Psu SLI gold or silver power. They will cost you but well worth it.
I was told today to find a psu with 2 12v rails for running 2 7870s. is that correct?
If you're looking for a multi-rail power supply and if it's properly designed for your specific system power requirements it should have at least four +12V rails.
If you want your system to be able to survive running FurMark ...
For a system using two Radeon HD 7870 GHz Edition graphics cards in 2-way CrossFireX mode a minimum of an 650 Watt or greater system power supply is recommended. The power supply should also have a maximum combined +12 Volt continuous current rating of 47 Amps or greater and have at least four 6-pin PCI Express supplementary power connectors.
Many of Antec's models are multi +12V rail designs.
Most of Corsair's and Sesonic's models are single +12V rail designs.
All of XFX's models are single +12V rail designs since they're all based on Seasonic platform models.