Using a shared DVD burner
I think that feature is only for sharing discs that are in the drive, not for sharing the drive and it's burning capabilities itself. The manual says:


  If you have installed the DVD or CD Sharing Setup software, you can share DVD or CD discs in your computer’s optical drive with other computers on your Ethernet or wireless network. If another computer on your network doesn’t have an optical drive, you can set up your computer to allow the other computer to use the disc in your computer’s optical drive. 


Instead of what you're doing now, try saving from iDVD as a disk image. You can then use Disk Utility on the other mac to burn that disk image to a DVD-R.