I suggest using doggy doors and 15-25' long by 10'x15' wide kennels, with concrete base. I use the doc-bob professional kennels purchased through Tractor Supply kennel for all my newer kennels.
The beauty of the panels is that, if your dogs are good together, then you have the luxury of removing the panels in between and giving them a larger, joint area. If they are not, then your dogs will be safe from each other. But you can always add them back in.
It's nice to protect the siding with fencing. Some GSDs will eat the siding. It is also good to have something solid on the inside. You could use an x-pen -- 4x4 and cover it if dogs are likely to fight. Or, you can use the doc bobs inside. If you turn them on the side, they are 6' long by 5' tall, so you could use a 6x5 or 6x10' area.
Another idea is to have baby gates, and give the dogs a room inside and just baby gate them into that room. The metal extra-tall baby gates with no horizontal bars, keep my dogs in. They respect the baby gates, and while I generally coral them so that they have more than just one baby gate between them, even when that is the case, I have never had issues.
Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.