Hi eyeV,
You can load the EXOS image on both peers and install it, then reboot one peer and wait for MLAG peer to get in sync/up. After that, reboot the other peer.
I had to do that (in a MW) for about 30-40 MLAG pairs (60-80 units) and worked without any issue.
During the process, the peers will be temporarily in different versions and that won't be an issue.