Quote How do I merge two movie clips in PD? Combine option is greyed out cause they are from different sources and not from bigger clip that was split.

You can't, on the timeline, but no real need to. Produce your timeline and you will have one combined clip.

Jeff

As far as I understand the COMBINE option is to combine two pieces of the SAME clip after that clip was split and no frames were removed since the split. It is in fact to undo the split.
See: https://help.cyberlink.com/stat/help/powerdirector/365/win/enu/08_01_00_splitting_a_clip.html?q=combine

Please explain why you you want to merge two (different) clips?

If it is to get one video-clip from two: do EXPORT and write the clips to a video file.
If it is to move the two clips around: Select both clips and in the right click menu select group. After that the clips will be grouped together when moved around, deleted etc.

Otherwise: Please explain.

Quote
Please explain why you you want to merge two (different) clips?

I need all of them to have one common "loudness line" in audio.

Alternatively is there a way to normalize audio in each clip to certain peak level? Right now normalization is greyed out unless I click two clips and then the audio is normalized not to peak 0,0 but to the level of the quiter audio.

Quote I need all of them to have one common "loudness line" in audio.

Alternatively is there a way to normalize audio in each clip to certain peak level? Right now normalization is greyed out unless I click two clips and then the audio is normalized not to peak 0,0 but to the level of the quiter audio.

You can’t set a peak or anything during normalization. However, after normalization of the two clips together you should have about the same loudness. Then one can adjust the whole track gain to boost overall volume of the track, both clips will increase. You make this adjustment in the Audio Mixing Room with the horizontal audio gain slider for the track of interest.

Jeff