Why can't I get better than 1gb performance bonding NIC's on my ReadyNAS Pro 6?

When you use LACP each data flow is assigned to one of the output NICs - that's done by the hash choice (layer 2, layer 2+3, layer 3+4).  Which ever option you pick, you end up with a coin-flip for each flow.  The outcome isn't random though - for a given output flow between two devices, you'll always end up with the same assignment.

In your case, the two iPerf flows happened to give the same answer for those two PCs, so both iPerfs ended up running over the same NIC.   One option is to try changing the IP addresses of the PCs (using layer 3 hash), and see if you can find IP addresses that give better performance.  That likely won't help in the PC->NAS direction, as most switches use layer-2 (and don't let you change that).

Another option is to try a static LAG on the switch, and use "round robin" on the NAS.   There is a disadvantage though -  the NAS will try to send > 1 gbps to a single PC user.  That will create packet loss in the switch.  TCP will back off the data rate to compensate, but it might unstable.   If you see that, you can also try enabling ethernet flow control in the switch.