A thermal differential like this is actually not unusual on many tower-style multi-CPU systems. In essence, outside (cool) air flows over the heatsink for one of the CPU's first (in your case the one in socket 0), and then over the other one. Because the air is warmer when it flows over the second heatsink, it ends up not absorbing as much heat from that heatsink, so it doesn't cool that processor as efficiently.
However, in yoru case, you've got an easier way to tell which socket is socket 0, namely just look at the motherboard. When you've got these complicated optional riser board configurations, the socket on the motherboard is always socket 0.
As far as temperatures, 85 degrees celsius is a bit high (Intel CPU's are designed to run hot safely, but 85 is really close to the thermal throttling point for most Intel CPU's), but I doubt that redoing the thermal paste will help much unless you get really nice thermal paste and make sure to properly clean both the package and the heatsink. Even then, I doubt it will have much direct impact (maybe 5 degrees if you're lucky). You're probably going to get much better results by getting a better heatsink or improving the airflow through the case.
Is everything allright?
prochot, btw on mine is 92°C.