Your power supply probably isn't adequate for 2 4850s. It only comes with 1 6 pin PCI-E connector, which is a huge red flag for an "850 Watt" unit. A real 850 Watt PSU should come with 4 6 pin PCI-E connectors. I would say that the amperages listed on the PSU's label are an outright lie. That is probably a 350 to 400 Watt unit at best.
Get a new power supply if you want to run two 4850s. With your current setup you run a high risk of blowing out your power supply and taking your components with it.
Some good power supply brands include Antec, Corsair, Seasonic, and XFX. I recommend a 750 Watt unit for 2 4850s, those things consume a lot of power, 187 Watts each.