  Quantum Random Access Codes (QRACs) are key tools for a variety of protocols
in quantum information theory. These are commonly studied in
prepare-and-measure scenarios in which a sender prepares states and a receiver
measures them. Here, we consider a three-party prepare-transform-measure
scenario in which the simplest QRAC is implemented twice in sequence based on
the same physical system. We derive optimal trade-off relations between the two
QRACs. We apply our results to construct semi-device independent self-tests of
quantum instruments, i.e. measurement channels with both a classical and
quantum output. Finally, we show how sequential QRACs enable inference of upper
and lower bounds on the sharpness parameter of a quantum instrument.
