Thanks MJ. Good advice and that's pretty much what I told her for both songs she has done so far. I stated the following prior to her recordings "Please feel free to change it and sing it that sounds best to you. Looking forward to your tracks."
I also told her prior to her recordings that this song will feature her vocals only. I only added my vocal tracks afterwards out of curiosity and that's when I noticed she sang it at a higher pitch to try and match mine. Prior to her recordings I sent her one instrumental for her to record over and one with my vocals on it as guide to how I chose to sing it. Airgigs even asks to have a guide track sent along with an instrumental and bpm number.
The following is what I sent her after I listened to all five of her versions she sent.
"I appreciate that you sent several versions. Unfortunately, I am asking for a revision, please.
For this song I would prefer a more relaxed approach similar to how you sing in the video (below)
I'm completely okay if you want to sing it in a lower register as in the way you sang Still, No Change. Maybe try a version similar to my singing pattern (but not my voice) The phrase "Sometimes less is more" comes to my mind. I hope I don't come off as sounding critical, because I am not trying to be. I do understand the approach you took for this more up-tempo song."
A later message from her "Hi Nick!!Yes it was really high for me, but when I was following your same pitch, it was too low and it wasn't good for the song. I listened to the mix and our voices sounds really good!!! the thing is if you want our voices together, maybe I can record it again but following exactly your phrasing, Its up to you, if you are happy with what we already have."
My reply "When you previously stated (prior to your recordings) the pitch was too high I wasn't sure what you meant. However, now that I know it was in contrast to my singing it makes sense. I think that your (her) idea to record a new track with the phrasing I used is worth a try because it can serve a dual purpose for one version with us together and another version with your vocals only. In saying that I think it would be good to start out your vocals close to mine and then build your range as the song progresses. I'll leave it up to you to pick those spots."
The song below I believe is her natural comfort zone even though she can obviously go higher and lower. My guess is that if she stays within this range, even at the tempo of No Second Guess and followed my pattern to start out with (eventually building range as the song progresses) we will likely end up with at minimum a better timed duet and a great solo track.