You could always ask for two entirely separate friends to BOTH review your work and have them not know who the other friend is (or for that matter, even that there WAS another friend reviewing your work, if needed.) They wouldn't be able to easily form an echo chamber because they would not be in communication with each other.
Alternatively, you could, if you really wanted to avoid an echo chamber, explicitly ask Poster A "Can you review this with the intent of finding flaws? Crocker's rules apply, be as harsh as you like." and Poster B "Can you review this with the intent of finding good points?" It does not seem like that approach could form anything remotely like what I would consider an echo chamber.
That's asking a fair amount from people I only know over the Internet. I could count on one hand the number of people that I expect a >50% chance of response (even refusal) if I asked them for this kind of help on this site.
And I'm not sure anything is wrong with that.
Several people posted recently in a thread on women, mostly espousing feminist views - only to find that someone had declined to respond to their post, but instead browsed their history and downvoted every single comment or article they had ever posted.
I have two questions:
1. Why would you come to a site like this and pollute the karma system? How does it make you smarter? How does it make anyone else on the site smarter?
2. What would be a good technical workaround? In my mind, some system that detects mass-downvoting and flags a user for review would be preferable, but what should happen then? Should the system be more lenient to higher-karma posters? Who should perform the review process? What should be done with those whom the reviewer ascertains are abusing the karma system? I would prefer some kind of lesson that is more corrective than retributive - it seems to me that people who would perform this behavior are exactly the sort of people who need some of the lessons that this site provides. Any ideas?