ATA is fine with two devices on a single connection normally. They need to be configured cable select (CS), or master/slave.
First, reseat all cables. This means disconnect and reconnect them.
Second, see if the BIOS sees the HDD and DVD. If not, check power cables to the devices, check ATA cable, attach to second ATA port, isolate devices (1 per cable). Thiw will rule out bad connection/power cable, ATA cable, ATA port, drives interfering with eachother on the cable. If you don't have two cables, try each alone on the cable. Don't forget to check cable selection jumpers on the drives.
If the BIOS sees the hardware and it still doesn't work, attempt isolating the DVD. completely disconnect the HD to see if the DVD will boot then.
If you still don't see the HW, swap with known good (bring a spare DVD/HD). If that doesn't allow the HW to be seen, it's a bad MB/port/ATA issue. You might try reseting NVRAM via jumper and praying.
Third, if you see the hardware in the BIOS and no boot then try to put just the DVD drive in. You won't be able to install, but you will be able to tell if the HD is affecting ATAPI booting. If so, check jumpers, ATA cable, swap DVD with known good.
Try booting to a floppy just to see what happens. If you have an old Win98/ME boot disk try sysing the HD and booting to the HD.