There was a difference between the hierarchy of ranks and the hierarchy of feudal positions.
The hierarchy of ranks might go from landless knight or man of arms to lord of a manor to count to duke to king.
The hierarchy of feudal positions might go from a fief holder who had no vassals of his own to his immediate superior overlord to his overlord's lord to his overlord's lord's lord, and so on up. People's position in that hierarchy depended on who they held fiefs from and who they granted fiefs to.
Similarly in the US army the hierarchy of officer ranks goes from second lieutenant to first lieutenant to captain to major to lieutenant colonel to colonel to brigadier general to major general to lieutenant general to general. Ten ranks.
But that does not mean that in the hierarchy of command positions in the US army each officer has to be subordinated to an officer one grade of rank higher than him. There are often more levels in the hierarchy of command positions than their are separate officer ranks.
For example, during the US civil War of 1861-1865 regiments were grouped into brigades that were commanded by colonels or by brigadier generals, brigades were grouped into divisions that were commanded by brigadier generals or by major generals, divisions were grouped into corps commanded by major generals, corps were grouped into field armies commanded by major generals, field armies were sometimes grouped into larger units commanded by major generals, and all units were part of the US army commanded by a major general until U.S. Grant was promoted to lieutenant general in March, 1864.
So in the hierarchy of command positions in the US army during the Civil War three, or four, or even five different levels in the hierarchy of command positions were filled by officers of the same grade, major general.
In the feudal system, it was perfectly possible for someone to inherit, buy, or be rewarded with, several different fiefs and hold different fiefs from different overlords. It was possible for two lords to each be an overlord of the other lord, for different fiefs. It was possible for someone to be his own overlord.
Acquiring another fief could often complicate someone's feudal relationships, but nobody would pass up a chance to acquire more land and wealth merely because it would make his web of feudal relationships more tangled and harder to visualize.
My answer to this question:
What was the relation of Barons to Counts/Dukes/Earls in England during the medieval ages?1
has a discussion of the feudal hierarchy which may be useful.
In the hierarchy of ranks the kings of England and France were equals.
In the hierarchy of feudal positions the Duke of Normandy, for example, was a vassal and subordinate of the King of France. And the Duke of Normandy, etc. happened to also be the King of England for centuries.
England was a separate independent country from France, making the two kings equal as kings. The King of England was also a count and duke in France, and as such subordinate to the King of France.
If the King of France tried to make the King of England his vassal for England the same way the King of England was his vassal for Normandy, he would be committing aggression against the King of England.
If the Duke of Normandy, who also happened to have the other job of King of England, tried to make Normandy independent of France the way England was independent of France, he would be committing treason against the King of France.
My answer to this question:
Which European nation had the most kings in the 18th century?2
Shows that there was a period of a few months in 1762 when the rulers of 10 European states outside of the Holy Roman Empire were also the holders of fiefs inside the Holy Roman Empire.
Thus those ten rulers were both rulers of independent states outside the Holy Roman Empire and also subordinate vassals of the Emperor for those fiefs inside the Holy Roman Empire. As my answer shows, similar situations had existed for centuries, and those months in 1762 were only unusual in having as many as ten rulers in that situation.
So it was quite normal for a king or other ruler to also be a vassal of another king or ruler for part of their lands, and in most cases being a vassal of another ruler for some of their lands didn't cause nearly as much trouble as it did in the case of the Plantagenet lands in France. Presumably because most persons were able to act differently as a vassal for some of their lands than as a monarch for their other lands.
It seems that some of the kings of England and of France were unable to function differently in different relationships as well as many young children manage to do, and thus there was trouble between England and France over the fiefs which the English king had in France.