Let's try to construct some examples - I need to clarify this as well. Can you please tell me if any of the examples below are wrong.
1.
I have a Province (4), in which I have two holdings: Law (4) and Manor (4). For an Agitate Action Advantage (Law) and Advantage (Manor) applies.
I get a +6 bonus to my Agitate Action. +4 from the law (or manor) holding and +2 for the other holding (because it's bonus equals my current).
2.
I have a Province (5), in which I have a Law (5) holding. Again I am Agitating. Additionally I have a Manor (3) holding and an ally is helping me with a Guild (3) holding.
I get a +6 bonus in total. +4 from the Law holding, and since the Guild holding and the Manor together equal or exceed the number of levels giving me +4 bonus, they give me +2 synergy bonus.
3.
I have a Province (5), again I am agitating. In that province I control Law (5) and Manor (3). Additionally, an ally is helping me with her Guild (2) holding.
I get a total +6 bonus for the action. +4 for the Law, +2 synergy from the Manor and Guild because the total levels of assisting holdings equal or exceeds the number of levels providing the +4 bonus.
4.
I have a province (6). I am Agitating. I have a Law (4), an Ally controls a Law (2) and a Guild (2), he is helping me with both holdings.
The total bonus is +4. +2 from the Law holding, +2 synergy from my ally because the total number of levels he is helping me with equals the levels providing my +2 bonus.
An important question:
Option a) To gain the +2 synergy bonus in addition to the highest holding giving me advantage does the level from the other holdings need to equal or exceed the holding levels giving me advantage? This is how it is written in the regent guide pg. 75.
OR
Option b) Does the advantage I would gain from each holding need to equal the advantage I gain from the highest level holding to give me +2 synergy?
This becomes important if I eg. have a Law (5) providing me advantage and getting advantage from from a Guild (2) and a Manor (3) holding, all in a Province (5). Option a) would give me a total +6 bonus. Option b) would only give me +4, since the Guild holding is too small to provide advantage and the manor only provides +2 (less than my +4 bonus).
I have a feeling the intent might be option b, but personally I like option a better (slightly simpler).