It's regarded as a new order, but if you already have the same aircraft on order, it adds it to the existing one. I rely on this all the time, as I keep my orders low early in the game (you usually can't afford much) & then add on to them before the last one is delivered. I just tested it myself - I have 6 A300's still on order, which has a 6 week wait period for the first one & then delivered every 3 weeks after that. My last delivery in my game world is on May 1. When I placed an order for one more (leased), it advised delivery would be May 22, which is 3 weeks. So it's adding it to the existing order & still delivering on the 3 week cycle, not making me wait 6 weeks.
So to answer your question from your example, your first A319 of the second order will arrive 2 weeks after the last delivery of the first order.
However, if you have A319's on order & decide to order 5 A320's (different aircraft in the same family), it will make you wait 4 weeks after the last A319 is delivered. Have a play with it yourself, you can't break anything.