I think the SCPH-7000x models are V13 Slims and you need to use special connections for a HDD, like HD Combo and a Modchip to get them working properly.
Do both those have a modchip? And do you already have a HDD connected to both?
Still not sure how to give you advice without knowing what modchip you have. Not all modchip function the same and giving you wrong advice could make things worse for your console.
If you dont have a modchip in the other one, then you might be able to use Free McBoot as a modchip replacement. You simply use the modded one to run a CD installer and install it to a MC, then place the MC into the non-modded console and you will be able to run HDLoader. But you will also need to connect the HDD the same as the other one is.
Ya bof them are modded and on one console the HDD prepared by a Professional is working also I just want to prepare second HDD (80GB IDE)(Which I was using in my PC before) for the second console....
ok, then connect the HDD in the PC and run Winhiip to format the HDD for PS2 usage. Once the format is done, you can then start installing games to the HDD right from your PC. Choose to install from disc drive or "Image" if you have ISOs on your PC already.
Then just put the HDD into your console and select HD Loader from your menu. If you dont have it on the console you want to use this HDD with, then you can burn a HD Loader to CD and run it from there.
Since i still dont know what modchip you have, i cant advise on how to install HD Loader to Dev for you. You could look thru some modchip sites and see if any of the features/options or screens (if they show any) match with yours.