Removing that echo will be tough, to impossible.
You have frequencies, that overlap, so if you address the frequencies, you'll be taking down both the original and the echo.
For general background noise, there are tips in this ARTICLE, but I really do not think that anything there will work.
Maybe some heavy-hitting audio guru will teach us both something. If so, I will gladly add their instructions to the article above.
I think that you might be in for a re-shoot, or living with the audio, and altering the micing techniques next time.
PS - Welcome to the forum, and I would also post to the SoundBooth and to the Audition forum. For the latter, ask for general techniques for working on such in Audition (do not mention PrPro, or SB), and then we'll see if they apply in SB.
You might try using the Expander in the Dynamics plug-in. It's not a very good one since it doesn't have ADSR (attack, decay, sustain, release), but it might help a little. Try setting it to -20 with a ratio of 1.5 o 1.75. You also might need to adjust up or down from -20. I don't hold out a whole lotta hope that this will help, but it's worth a try.
Did each mic record to separate tracks (hopefully) or were they both mics to both tracks( unfortunately).
If you got lucky....try
Deleting one of the Tracks using either Fill Left or fill Right
It was definitely the unfortunate situation you described. I did try the fill left and fill right tracks, but the echo was present in both. Thankfully its only on one segment that the echo is most pronounced. I'll definitely be more careful next time. Thanks for the help!
Glad to hear that it was somewhat limited in your Project. Even with a full-featured DAW, and hours to fiddle and listen, such an echo is really, really tough. It's not too different from shooting and recording in an environment, where there is music w/ vocals in the background, and trying to remove that from the voices that one wishes to keep. A hint of cleanup might be "as good as it gets."
What you hear as an "echo" is probably a tad of latency between the devices and also the distance from the speaker.
Such is why I try to use very narrow field mics, and only record in mono. The greater the isolation of the Audio sources, the easier it is to address later on.