They will cruise together, propelled by a third module MTM until they are put into orbit around Mercury.
However, sending out such a mission involves major technical challenges. Being so close to the Sun will expose the probe and its instruments to a very high level of radiation and temperatures ranging from -180°C on the night side to +450°C on the day side. These extreme conditions will require the implementation of new instrumentation concepts, such as PHEBUS (ultra-violet spectrometer) or SIMBIO-SYS.
After 18 years of development, the probe was launched towards Mercury on October 18, 2018. It will not reach its destination until 2025.