Don't know if this is still going to help you, since it's been a month, but this thread comes up first when I search for mythic summoner advice via google, so here's what I've determined so far (haven't put anything into practice yet)
I've been looking over the different options carefully since I found out this stuff was out. As I read the dual path feat (which could be wrong), you first get one of the three basic abilities from the other path, and then thereafter whenever you achieve a new mythic tier you can take path abilities from either path that you meet the prerequisites for (or from the universal path abilities list). Meaning if you were an archmage and took that feat at tier one for Marshal, you could choose a tier three Archmage or Marshal path ability once you reached tier three. You'd also get to pick one of the Marshal's Order powers when you first picked the feat.
Now, at this point the main thing you'd want to look at are the tier 10 powers that each path grants (assuming you expect to get to tier 10) and how much hp per tier they grant. Whichever seems better for your character should probably be the primary path. As a summoner you'd probably want to go for the one with higher hp/rank (Marshal in this case) since dual path doesn't grant hp for the second path.
So I'd say archmage should be the secondary path gained through Dual Path. The 15+ character level spell resistance tier 10 archmages get is nice, but it only works against arcane spells - divine spells can still screw you over. You can also get 11+summoner level SR against all spells with Greater Aspect. Meanwhile the Marshal's tier 10 power of getting a use of mythic power back whenever allies close to you score a critical hit could work really well for you regardless of your build.
As a side note, you can take a normal feat in place of a mythic feat if none of the mythic feats are available or more suitable for your character.