The problem you're having is in attaching the final clause:
NAME is...that helps...by rating...and helps... .
When the reader gets to the "and" he's expecting it to bind to the "by" -- NAME helps by doing two things, rating and...helping. But the next word is "helps", which doesn't fit that pattern. So the reader has to mentally backtrack and conclude that instead of:
Clause (that helps by (A and B))
he's looking at:
Clause that (helps by A) and (B)
This can be jarring anywhere, but it's especially disruptive in technical writing where the writing is usually more precise and careful than in, say, fiction. The readers of technical documentation sometimes also read specifications, requirements, interface designs -- all areas where precise bindings of phrases and specific word choices matter. Now in the case of your example that binding question is only a distraction; it would be more severe if you instead had something like this:
Perform an emergency shutdown of the nuclear power plant by frobbing the widget and twiddle the doohickey.
Is "twiddle the doohickey" part of the process of shutting down the power plant, or is it a separate, later step? This is the structural problem in your example, though I trust that your readers will have fewer safety concerns.
If you mean to say that the community helps its members code better by (something else and) helping managers pair with fellow developers (i.e. helping managers is part of making members code better), the minimum change to make that work would be to change "help" to "helping", for parallelism with "rating". But I'd massage it further; the double "help"/"helping" feels repetitive, and arguably you'd want a second "by" because of the length of the first clause.
Your first version presents two clauses with equal intended weight -- two ways that the community does what you say it does. Your rewrite makes one of those reasons primary and one secondary, by moving the latter into a new sentence and saying "also". If the manager part really is secondary, then your rewrite is fine. If both are meant to be equal, though, try splitting it up differently, putting both reasons together in the same sentence:
NAME is a community that helps its members code better. It does this by having members rate each other and by helping managers to pair with other developers.
I'm focusing on the structural question here; you might want to word-smith this further.