Trouble with luminosity masks

Ah, now I get what you're doing. This used to be a fairly standard technique for highlight recovery - basically using the shadows from one exposure and the highlights from another.

The reason this doesn't look good is simply that the two exposures are too far apart. The net effect is that the upper midtones "melt together" and crush into a drab gray. There's not the local contrast you need for a credible result.

I never found this technique to work out of the box, mostly for this reason. You need to do some additional local brushing to tip the balance towards one or the other. Bring out the dark exposure in the highlight areas and vice versa. Use a soft brush at low opacity - just skip the luminosity selection and paint carefully with black or white, slowly building up the local contrast.

Today this technique is less used (at least by me) - camera sensors have vastly extended dynamic range, and ACR highlight/shadow recovery is far more effective than it was in the early versions. Nowadays I can usually get what I want from a single exposure.

It's unclear what you're trying to achieve here.

Here's a luminosity mask, which is normally used on adjustment layers. Where does painting come in - are you trying to modify the mask or the image with this painting?

Could you please post screenshots taken at View > 100% and with the pertinent Panels (Layers, Channels, Options Bar, …) visible to illustrate what you are talking about?