Sounds like this is a dedicated mining rig. Go in your BIOS and disable EVERY non-essential piece of hardware that you wont be using. Audio. Firewire. All but 1 NIC. Serial ports. Parallel Ports. Serial ATA Ports/controllers except 1 if using a hard drive. All but 1 CPU core if using a multi-core CPU. USB ports, unless you need a keyboard to get in for something. All of that is consuming "resources" I'm assuming that can be used for the hardware you actually want to use, i.e. video cards. Also, using 16x > 16x risers may be causing a problem, because your board is trying to negotiate to 16x speeds. Some boards, when negotiating to max slot speed, will turn off other slots entirely in order to have the full amount of bandwidth available (PCI-E lanes are not unlimited). Try using 1x risers across the whole shebang. If you have limited 1x risers, use the 16x risers in your PCI > PCI-E adapters since they are always 1x speed.
Also make sure your risers are not defective. I had a bunch of 1x/16x > 16x risers, and they all went bad because of the stress of plugging them into the cards or bending the cables, would either cause cards to freeze the system when a load was applied, or device manager would show errors, or the cards wouldn't even detect at all. I use 1x > 1x risers with the edge sanded off in order to accept anything bigger than 1x, and they have all worked no problem (have 10 in use now), and are effortless to plug a card in.
Edit: I didn't really read much of the article before. You say 16x > 16x doesnt work, but 1x > 16x does. It could be a bad riser or bad speed negotiating. Use the 16x> 16x risers in your PCI > PCI-E adapters since they always work at 1x. Powered shouldn't matter if you are doing underclock/volt. Also, I already have experience with 6 cards on 1 board as well as using PCI > PCI-E adapters (I'm using one in a 4 PCI-E slot board now, now have 5 slots).
Oh, almost forgot. You say you are using 2 PSU's. If you can, power on the PSU that ISN'T powering the motherboard FIRST, then the one that does SECOND. I mean, kind of worth a shot I guess. I've forgotten to plug in PCI-E power cords to a card before and they wont detect, or they malfunction. If the card doesn't have proper power before the motherboard tries to detect it, that might be a possible problem too.
Also make sure your risers are not defective. I had a bunch of 1x/16x > 16x risers, and they all went bad because of the stress of plugging them into the cards or bending the cables, would either cause cards to freeze the system when a load was applied, or device manager would show errors, or the cards wouldn't even detect at all. I use 1x > 1x risers with the edge sanded off in order to accept anything bigger than 1x, and they have all worked no problem (have 10 in use now), and are effortless to plug a card in.
Edit: I didn't really read much of the article before. You say 16x > 16x doesnt work, but 1x > 16x does. It could be a bad riser or bad speed negotiating. Use the 16x> 16x risers in your PCI > PCI-E adapters since they always work at 1x. Powered shouldn't matter if you are doing underclock/volt. Also, I already have experience with 6 cards on 1 board as well as using PCI > PCI-E adapters (I'm using one in a 4 PCI-E slot board now, now have 5 slots).
Oh, almost forgot. You say you are using 2 PSU's. If you can, power on the PSU that ISN'T powering the motherboard FIRST, then the one that does SECOND. I mean, kind of worth a shot I guess. I've forgotten to plug in PCI-E power cords to a card before and they wont detect, or they malfunction. If the card doesn't have proper power before the motherboard tries to detect it, that might be a possible problem too.