Dear context group members,
The 2UP arranging spoils the table of contents and the index. Deleting the second rule gives a content and index, including the second rule gives only the titels 'content' and 'index'.
\setuparranging[2UP,rotated,doublesided]
\starttext
\completecontent
\chapter{hfdst 1}Dit is een verhaaltje.\index{verhaaltje}
\chapter{hfdst 2}
\chapter{hfdst 3}
\chapter{hfdst 4}
\completeregister[index]
\stoptext