Tandem exon derived substitutions involve the swapping of one (or more) coding exons for homologous coding exons. These may be at the 5' or 3' end, or be produced by mutually exclusive splicing. These transcripts produce two homologous proteins (or more) from the same gene.
These genes are from GENCODE v33 (maybe they are not valid for any other annotation).
These data can be downloaded here in TSV format.
There are two papers detailing these isoforms:
Laura Martinez Gomez , Fernando Pozo , Thomas A Walsh , Federico Abascal , Michael L Tress
DOI:10.1093/nar/gkab623 Origins and Evolution of Human Tandem Duplicated Exon Substitution Events
Laura Martinez-Gomez , Daniel Cerdán-Vélez , Federico Abascal , Michael L Tress
DOI:10.1093/gbe/evac162