This guy, Ambrosio Spinola, was a Italian mercenary fighting for Spain in the early 17th century.
And *this* guy, Frederick Henry, Prince of Orange, was a Dutchman who fought *against* Spain, also in the early 17th century.
This is clearly the exact same same fancy armor. They seem to be wearing similar necklaces. The helmets seem similar but clearly not the same, but they have the same fancy plumage. There's clearly some amazing design work here. Is it actually literally the same armor but different helmets? They also have the same stick in their hand...but that's pretty easy to replicate. Did one guy capture the armor from the other the other and is showing it off? Does this armor have a name? I know these guys were mercenaries. But maybe the painter Michiel Jansz van Mierevelt, who painted both of these portraits, just told em both, "Hey, show up on Saturday and I will paint a neat portrait of you. Don't worry about wearing armor; I'll add it in post-production!"
What is going on here?
The most likely occurrence that there may have been, is that both of them being being of very high status (Spinola was a Genoese nobleman, grandson of the prince of Salerno and son of the marquis of Venafro, and Frederick Henry was the sovereign prince and Stadtholder of five of the territories making up the Seven Provinces. You can hardly go higher than that), thus being very, very wealthy, purchased the best protection they could possibly afford and decorated it according to both their tastes but also, in a manner, fashion of the time.
Additionally, these two persons most likely obtained their armours by blacksmiths either from Milan (which at the time was a Spanish territory), very famous already in the closing centuries of the Middle Ages for being a plate harness production center (to the point of spawning or at least naming a model of plate armour, named "Milanese style" a seen here in the equestrial monument for Italian mercenary captain Bartolomeo Colleoni (1395-1475) and in one of its evolutions here with the potential armour of Ottavio Farnese, duke of Parma (1524-1586), or from those armourers influenced by that style of craft, which possibly were in Spain by this time period; I once were told of a postulated "Spanish style" of plate armour but I do not know enough on the matter, so take this with a pinch of salt.
For Dutch prince van Mierevelt's painting, we see the next step in the evolution of said armour which some call "three-quarters armour" due to its tendency of covering the wearer from the shin upwards, leaving the lower leg exposed. These models can be seen at the turn of the XVII century, with some early iterations towards the closing years of the 1500s, as these two examples, which are most likely Savoyard in origin and similar to what the mounted soldiers composing Emmanuel Philibert (1528-1580) Duke of Savoy's personal bodyguard could have used.
Lastly, as you have suggested, the painter may have taken a couple shortcuts and reused some common aspects of the armour that he may have painted in other representations, but I doubt that the bulk of the illustration was made up.
