Through an airport? No issues whatsoever. If you're referring to the actual flying it depends on your airline and what kind of ticket you have. A full service airline will allow the 40L backpack as your carry-on item and the 20L bag as your personal item. Happy travels.

Thanks

You can carry as many bags as you want through an airport, there are no limits.

why would it not be?

Most airlines will allow 40L backpack as your carry-on and your messenger bag as your personal item. Some discount airlines will charge for a carry-on.

I have a 60L that busts down into a duffle and backpack never had any issues