Don't worry about the level as much as the blend. No two voices are alike. It is the blend of the unsions and harmonies that are what make up the color. Unless its death metal and then all bets are off.

Light compression, may well help.
Very light.

ya could try use a noise meter, get em around the same levels keeping note of their peaks,,,
but never really did anything for me, but you could help ya

a little compression to even them out and some panning for separation. The separation should help a bit even if the levels arent exactly even.

Is this a fairly simple duet where you can regard one of the vocals as lead and the other as harmony? The lead could be passed between vocalists from phrase to phrase or section to section, but I assume we're not talking some Bach fugue where two lines combine to form seven melodies.

If so, you want to try nailing down the level of the lead vocal first and then adding the harmony part in a supportive role. If the lead vocal is being passed around, you may want to go so far as to duplicate both tracks and edit them so you have (1) Vocal A lead (2) Vocal A harmony (3) Vocal B lead (4) Vocal B harmony. You could then start by working with just the two lead tracks so there is a consistent level of lead/melody. Then add the harmony in support of the melody.

I guess what I'm saying is rather than trying to "balance" the two lines perfectly, you make a choice of the "main" line and then mix the second line in a way that makes the first sound best.

Just something to try when all else has failed.

As a vocalist I've sung a bunch and recorded a bunch of duets. Like most things GIGO - (garbage in garbage out).
Make sure the two can sing their parts well and get a relatively good balance acapella. If you're there then first choice for me is a 414 in figure of 8. Have the two face each other and go at it.

It may be best to have them both back off a bit and really work to make one sound and one phrase.
Light compression after the fact can smooth over a few bumps. FWIW - I never like to edit vocals a lot. Get it good then edit a bit or else keep rolling til you get it.

YMMV

If you're doing 2 seperate takes, then make scratch recordings for both singers to practice with so the phrasing and line can be worked out in advance.
Phil