Originally Posted by seoulmanjr
I've taken my hiking backpack as my carryon plenty of times -- all over the world and frequently on NW. It isn't an external frame and I compress it down to a reasonably sized pack with the straps on the top and the sides. Basically, do a reality check once you've packed it at home -- can you fit it in the overhead easily? Is it any mroe ridiculous than what other people usually take on the plane? And be prepared to gate check it just in case, though I've never had to.
peace,
~Ben~
The OP is wanting to take a backpack in addition to another carry-on however. Presumably the other carry-on is large and needs to go into the overhead as well, so his backpack needs to fit under the seat in front of him.