If an agent says they only respond if interested in an email query

Unless an agent specifically mentions they want to know if you're simultaneously qerying, I wouldn't mention it. Most US agents will expect you're doing this. (My understanding is things can be a bit different in the UK.)

If the agent does say she wants to know, simply add the line, "As per your guidelines, I note this is a simultaneous query." Then simultaneously query your heart out. There's no reason to sit around and wait for their (non) response once you've met their guidelines.

Cheers.