Well, I'm going to slightly depart from my usual characteristic response...
How did the iMac come to have no OS, and why don't you have its system restore DVD? If you lost / disposed of the system restore DVD, you nonetheless still own a license for the version of OS X that was previously on that Mac. I guess even, in principle, if the system restore disc had been given/sold to another user who was actually able to install it, that person too already had a valid license to the same version of OS X.
Assuming then that the eMac has the same version of OS X that the iMac shipped with (probably Tiger?) and it's fully updated, there's a destructive way to accomplish what you want:
1) Install SuperDuper on the eMac
1.5) This is probably optional, but I recommend it. Create a new administrator user on the eMac and don't log into it.
2) Bring the iMac up in target disk mode and attach it to the eMac; it should mount as a disk drive.
3) Clone the eMac drive to the iMac drive using the settings to erase the iMac drive before cloning (don't use the disk image setting or anything like that). This is the destructive part; you'll loose any data on the iMac.
When (3) is done, the iMac drive will be a bootable clone of the drive in the eMac. Since they're both PPC Macs, OS X should be willing / able to use this installation. For safety sake, I would boot the iMac off its drive at this point and log in to the new administrator account you created, as it will not have any settings that could interfere with your iMac hardware. It should autodetect everything hardware-wise and work properly.
Under any other circumstances, though, this is illegal. Using this technique, or any other, to run more copies of OS X than you have licenses is illegal. Even here it's on the darker end of grey.