It's very common that you need to brighten dual iso images in post.
You're saving the highlights with the low iso and have enough detail and less noise in the shadows because of the higher iso.
BUT after the conversion, the images always look (brightness wise) like images made with the lowest iso.
In lightroom, with most dual iso images I move the exposure slider to the right (2 or 3 stops, sometimes even more), I pull the highlights back with the highlights slider.
And I move the shadow slider to the right.
This will of course add some noise, but not as much as would have with a normal image (try it out, take 2 exact same photos, one with dual iso and one without and go to lightroom...)