It’s not a fully working system yet, I agree, but the principal and basis for multiple terminals is there at least and with @jasperwillem ‘trick’ you can separate check-in over different buildings as well. But would indeed be very nice to have the ability to designate different terminals in the game and connect public transport, etc. so PAX know where to go. Hopefully with assigning terminals the game will also automatically separate secure zones, or even better besides terminals designate different secure zones in area’s. There is still work to do and much to look forward to, but for now people can play around with the separated secure zones and create airports with at least what looks like separated terminals 
Will lower my enthusiasm a bit in the opening text, because I agree this is not a fully working multi terminal setup.