FAQ: How does dual indexing work? 

For paired-end flow cells, dual indexing now requires 23 additional cycles of sequencing - eight cycles for the Index 1 (i7) Read, eight cycles for the Index 2 (i5) Read, plus seven non-imaging, chemistry-only cycles at the beginning of the Index 2 (i5) Read. For single-read flow cells, dual indexing only requires 16 additional cycles of sequencing - eight cycles for the Index 1 (i7) Read and eight cycles for the Index 2 (i5) Read.*

* Adapted from Illumina website.