The two slots will most likely both run at x8 anyway.
Note: The PCIEX8_1 and PCIEX8_2 slots share bandwidth with the PCIEX16_1 and PCIEX16_2 slots espectively. When PCIEX8_1 is populated with an expansion card, the PCIEX16_1 slot will operate at up to x8 mode; when PCIEX8_2 is populated with an expansion card, the PC IEX16_2 slot will operate at up to x8 mode)
You can't notice a 1% difference in gameplay with the naked eye you would need an instrument to measure the difference in performance. Most motherboards are made for the shift from x16, to x8 and x8 and the performance dosen't suffer.