Just stumbled on this article: http://dogcare.dailypuppy.com/mean-dogs-bite-other-dogs-neck-2845.html

What particularly caught my attention was this statement:
Other signals of overt aggression could include: placing a paw or chin on the neck or back of another dog and holding it there,..., pressing another dog bodily to the ground or roughly biting his throat or neck.

Archer regularly does this with Ami - I've remarked to Wayne that the way Archer puts his paw on Ami's shoulders and pulls him to the ground is remarkably like a Wild Kingdom video of a lion bringing down a gazelle. Ami doesn't seem to mind - he just rolls over and grabs a piece of Archer...they take turns chewing on each other's neck. The only time I have heard a yelp from either of them is when Archer tried to body slam Ami and was sent flying through the air! Do I need to be concerned here?

Amy,

I don't think you need to worry, they have been socialized with each other since they were young (from your other posts). However the article is lacking, it's not so much aggression, imo, as it is a dominance issue, young pup testing older pup in who is top dog. Miya plays best with our next door neighbor's husky, he's a year older. When Miya was a young pup he did all the paw on head, etc. roles have now reversed. She is bigger and now she does the paw on head, etc. I wouldn't worry unless growling turns from play growl to a hey stop it growl, or the roll to the ground with teeth to back of neck and they stay in that position. Sounds like they are rough housing it. Archer is getting bigger and he is testing his boundaries.

Both my two do this to each other all the time. When we first got Keno he wasn't sure about kenzi's play style but has since picked up on it and now plays the same way. They drag each other around by the neck, smack each other in the face, body slam each other, all while making the most horrifying growling bear noises. I think what to look for is the ability to know when enough is enough. If one yips and the other completely disregards it, then you may want to step in. While one might not be meaning to hurt the other, it needs to learn that the yip means pain which means stop. Kenzi is a little slow on this still. She has a higher pain tolerance than Keno and he might yip if she bites a little hard so I would get onto her to break her attention every time I heard him yelp. Now when he yelps she lets go, but they still put each other through the ringer.

I wouldn't worry about it, you'll probably see their behavior change as Archer develops anyway. At the moment it sounds like Ami is just letting the kid away with it, he will start to hold his own and not roll over so much as the little one becomes a big un.
Orion's best buddy is a male Briard (wallace) my mums dog who is about 5 months older than Orion and honestly when they get together and start playing it sounds like they are going to war! It really is feral, they charge, pounce, chase, wrestle, bite, body slam you name it lol all while making horrendous noise then they back off play bow at each other and rush in again Orion is faster and stronger, Wallace is bigger and 15kg heavier so they are a good rough and tumble match, it's not aggression it's just boys being boys