Ok, create a new view, maybe a panes view. Add Album and Composer as categories.
In the Set rules for file display, press the Import / Export button and add (via copy/paste) the following:
[=1save(,composers_[album])]=1 [=1save(listcombine(load(composers_[album]), [composer]), composers_[album])]=1
Now, in the view's file list, add an expression column:
load(composers_[album])
and name it something like Composers_album (calculated). You should now see in this column your combined Composers list/Album. You can now use that load() expression to assign to your own Album Composers field by selecting all the files, and in the Tag Action Window, editing Album Composers and pasting:
=load(composers_[album])
Now your field has values you can use. Save this view for later updates, and maybe a reference to this post.