best is to update your tags
No way, it's not two (or X) separate albums. If you report your playback stats to last.fm or something like it, it's not very friendly to pollute their stats with your mistagged albums.
Try %album artist%[ / %album%[ CD%discnumber%]] and make sure your discnumber tags are set properly! (BTW you don't need $if2(%album artist%,%artist%) -just %album artist% does the same thing)
This makes a completely separate group for each disc in a multi-disc album, and that may be what you want, nothing wrong with it. I like that NG Playlist lets you have subgroups, so to utilize that, you could define two groupings like so:
$if2(%album artist%,$rgb(64,0,0)<no artist>$rgb()) - $if2(%album%[$rgb(64,64,64): %subtitle%$rgb()],$rgb(64,0,0)<no album>$rgb()) '['$if2(%date%,$rgb(64,0,0)<no date>$rgb())']'$tab()$if2(%genre%,$rgb(64,0,0)<no genre>$rgb())[ | %codec%]
[$rgb(64,64,64)%disctitle%$rgb()]$tab()[Disc %discnumber%/%totaldiscs%]
%subtitle% and %disctitle% are my custom tags, you'll probably want to remove them and their rgb() calls. Left them just to illustrate the flexibility. The other rgb() calls are there to highlight missing metadata, so hack away at that. And, if you don't want the "<no whatever>", none of the $if2() calls are necessary.
Keep in mind that absolutely any difference in either of these formats from song to song makes a new group or subgroup, so your tags have to be pretty clean. Also, this does make EVERY album group header two lines tall, even single discs, which might not be what you want. But, for mutli-disc sets, you can click at the very top to select all songs on all discs, or just above each disc to only select the songs on that disc. I really like that, musicmusic, good work!