  In this contribution we report on our work to increase the spectral range of
the Michigan Infrared Combiner-eXeter (MIRC-X) instrument at the CHARA array to
allow for dual H and J band interferometric observations. We comment on the key
science drivers behind this project and the methods of characterisation and
correction of instrumental birefringence and dispersion. In addition, we report
on the first results from on-sky commissioning in November 2019.
