Query Question: Revising novel when on submission to agents

I received an encouraging revise and resend with some great feedback from one of my top agents. I realized reading it that she was completely bang on in identifying some of the flaws in my novel, and began revising right away.

A little later, I received a full request from agent #2, whom I had queried some time ago. It's been about a week now and I haven't sent said full to agent #2, and I don't really want to until I've finished addressing agent #1's critique. But I'm worried that if too much time passes, agent #2 will wonder what's up. My manuscript was fully edited and ready to go (or so I thought) when I queried her. Should I let her know that I'm revising my manuscript based on feedback from another agent, or just send the full when it's ready (probably in another week or two) without an explanation? Is knowing that another agent has asked for revisions to a manuscript a positive or a negative in most agents' eyes? 

Here's the absolute ironclad rule: always send your best work. If you're doing revisions to fix some flaws in your novel, it's in BOTH our interests to have you send the better version.

Here's how you do this: You email Agent #2  RIGHT NOW and say "yo, snooks, got your request. Little late to the party, but ok for now.  I'm revising right now (I thought the novel was done!) and I plan to have it finished on X date. I will send it then unless you tell me otherwise.  Thanks again for your interest. I look forward to sending you my novel." Of course you put this in your own deathless prose. Remember to keep it SHORT. No more than 30 words total.

The thing to avoid here is a long period of silence. If I request a full, I pretty much expect to hear back promptly. That's because mostly I DO hear back promptly, not because it's critical to the submission process.

If I don't hear back within a week or so, my assumption is  my email went astray, and I email again.  In fact, this past year, I ended up emailing the writer who had REFERRED the querier because I hadn't heard back after several pings.

My colleagues are generally NOT going to do this. They're going to request and if they don't hear from you they're going to move on.  They'll remember you but they generally aren't going to track you down. I wrote a blog post that showed that a while back.