if I'm understanding your question correctly, you're asking if you do a second air attack after the first if you need to L-cancel the first.
Let me explain L-cancelling really quick before I answer this, you probably will be able to understand it this way.
L-cancels are the process in which you cancel the lag of an aerial when you land. Auto-cancels exist when you don't have to L-cancel (when you land and there is no lag followup). So in other words you'll want to L-cancel if you plan to land soon after an aerial with A button moves. In the case of B-button or special stuff like air dodges/grapples, I believe there are no known input cancellable versions. So something like Falco's SHFFL lasers or Fox SHDL will not cancel with L but will recover instantly, whereas marth's up-b will never cancel from an L-cancel.
If you want to do something like full hop, fair into dair (land) you only need to L-cancel the landing move, which in this case would be dair (provided you did it and didn't give it enough time to finish it in the air), but if you land immediately after the dair you will have to L-cancel it, you'll never have to L-cancel the first move because you aren't landing in between.
minor edit : L-cancels do not remove all lag, there is still some there. Autos I believe do.
There are exceptions to auto/L cancels, but yeah this is basic melee theory.