If you really don't want to go over ethernet (and I'm assuming fiber is out of the question, too), there aren't really any great methods to automate this process. If you don't want to pull drives and put your data at risk, you might be able to add a third drive to the mirror, let it replicate, then disconnect it and plug it into the second system. If you're on FreeNAS, check out this section for guidelines on how to add an extra drive to the array: http://doc.freenas.org/9.10/storage.html#replacing-drives-to...

...but really, you should figure out a way to go over ethernet. Even if it takes a really long time, it'll be soooo much easier to have everything automated. You also don't have to risk pulling drives, etc...