Most of the first wave of Intel H55 boards should be based on a single PCIe slot that supports external graphics card running in PCIe x16. While this is the norm, some companies like ASRock has models that spots two PCIe full length slots. The purpose of it is to support the running of CrossFire on the H55.
Although not in Dual x8 mode, the second PCIe slot will run in PCIe x4 supported by the H55 chipset. The other 16 lanes will come from the processor.
It is definitely a plus point to have such support but 4X with a 16X will certainly run slower than a Dual 8x.