One of them should do it. Keep trying a few times. If it doesn't work, Call compaq. Each one will support two devices. You may have to shop for new cables, to get enough length.
Extra length is ok, except bulky.
Giveaway at best. Each device on cable should be jumpered master or slave. Some hard drives come not jumpered, which is fine if they are the ONLY device on the cable. No special advantage to this. Doesn't matter if both hard drives are on same cable or not, but I would recommend making a second hard drive the secondary master.