Hi suthomas,
Greetings,
As per my knowledge, since you are upgrading a chassis cluster, you should add the image onto both the nodes and then issue the reboot command simultaneously on both nodes. The major requirement for the setup to be in cluster mode is having the same junos version, so issue the command simultaneously on both nodes.
Refer the KB for understanding this better: https://kb.juniper.net/InfoCenter/index?page=content&id=KB17235&actp=METADATA
Secondly, the no-validate would fasten your upgrade process but it does skip some basic checks which include the validation of software against the device and the current configuration on the box. If you have done this previously and are sure that there are no discrepencies with the s/w version and the device config then you could proceed with the no-validate option.
Hope this helps 🙂
Mark this as Accepted Solution if this answers your queries.
Regards,
Sharat Ainapur