One simplification might be an instruction to supply a
collective title if lacking, e.g., [Two Mark Twain novels], with
individual titles in whatever replaces 505 and 7XX.
That's an option under RDA for pieces that lack a single source of information (RDA 2.3.2.9)--effectively creating a collective title. LC and PCC catalogers generally won't do this, but that doesn't have to stop others from doing so.
