or does it seem to be a bad cable?

You didn't say whether a single drive on the middle connector worked or not. If a single drive works on the middle connector, then the cable might be just fine.

Other things to suspect:

- Not enough power to spin up both drives at the same time.

- Not enough spin-up wait-time in the kernel for both drives to boot. I wonder if someone can compile you a custom kernel with an extra-long wait time.

Has anyone else used this particular pair of drives successfully? Any tips to offer?