Can you legally have two wage attachments at the same time for two
Student loans in default in NJ?

Yes, as long as they don't go over the maximum allowed garnishment.

The first one would get the dollar amount it asks for, or percent, and if there is money left ( up to the amount they are allowed to take( that goes to the second one. Then when the first one is paid in full, the second one will start getting its full amount allowed)