As others have said, you knew their fab and billing policy up front - if you want their cheap boards, it is one design per board.
Otherwise send them 2x designs and pay an additional $5 or $10 for the second board, or panelize them like you did and pay them their 2x design in one board price. Or go elsewhere.
Any way, you are getting a fantastic deal - free cad tools, free validation tools, no up front cost to produce designs except your time, and effectively zero cost for the manufacturing process.
Stop calling bullshit because their attempts to run a profitable business run up against your attempts to squeeze more "free" out of their margins.
EVERYONE's cheap boards are a loss leader to get people to submit larger orders with them. If you disagree with their policies, make your own boards.