Does your area have limitations on how close the structures can be together? Not, you build one a couple inches taller than the other and have the eaves overlapped by a couple inches. One sheds water onto the other which would have a gutter.
As far as concrete or dry pack it depends on your soil. At my place I have a heavy clay expands and contracts considerably based on moisture content. I either pack with gravel or make a bell shaped also in the soil expands it does not push the post out. Wind is a factor as well. If you have super sandy soil it might not have enough holding power for what is essentially a big metal kite. In this case, you may need some substantial concrete puddings to secure it in these situations. Basically, the footing and anchor question hinges on your soil type. I just gave a couple of examples but there are a lot of variables.
As far as building materials go, I am a welder so I am partial to steel. I have a similar type structure though a little larger with rotting poles right now. My plan is to rebuild it soon in the same footprint to avoid the attention of the permit Nazis that do flyovers looking for unpermitted construction. I will use oilfield drill pipe for the poles and wide flange and angle iron for the spans.
