RE: .577 NE or .600 NE double rifle?
I wouldn't build either.
#1. Recoil (aproaching 50BMG levels btw)would be absolutely brutal.
#2. Because the recoil is so brutal the rifles would absolutely need to weigh at least15 pounds plus and would absolutely be a burden to carry around all day on a safari.
#3. Not only would the cartridges cost a whole lot of money you likely would not find any in the middle of Africa should they get lost on the flight over there.
#4. The only real good use for either of these two cartridges is to stop a charging elephant, rhino. Not a very good choice to "hunt" with IMO.