Two Portraits of Christian, Prince-Elect of Denmark, attributed to Pieter Isaacsz

The painting technique of two portraits presumably from the 17th century owned by the Museum of National History at Frederiksborg Castle are studied in CATS with the purpose of clarifying whether both portraits could be concurrent and painted by Pieter Isaacsz (1569-1625).
Publication or communication to a conference if paper is relevant to the field.