RU-6 MP-3 -> RU-7 -> RU-7 MP-2 on the first manager?
yes, thats correct
Can I just go from RU-6 MP-3 to RU-7 MP-2 once the first manager is done and the schema change is in place?
no the above thinking of your's is correct.
check this link
Migrating to Symantec Endpoint Protection 11.0.7200 (RU7 MP2)
http://www.symantec.com/docs/TECH187333
Migration paths
Symantec Endpoint Protection 11.0.7200 (RU7 MP2) can migrate seamlessly over the following:
Symantec Endpoint Protection 11.0.7101 (RU7 MP1)
Symantec Endpoint Protection 11.0.7000 (RU7)