Tunic split in the front an back are of course common in the period - being both practical for horseback riding and through their connection with riding and warfare also symbolizing manliness and knighthood.
Most of these tunics are, however not, shown with more decoration than a narrow line of white around the neck and sleeves, like these cute guys from Spain:
So I had to look around a little.
Matching embroidery at the neck, cuffs and the bottom of the tunic appears to have been most common:
You could also embroider along the split in the tunic. This is the Flight to Egypt from Ms K 26 at Cambridge University.
This guy from the Freiburger Münster has a narrow gold trim along the bottom and the sides of the split, as well as around the armscyes and neck, on his lovely striped surcoat.
Photo by Uli Frömmich.
The painting is new, but made after previous painting. One should probably note that he symbolizes a sin, so this is not a style for the virtuous among us :)
The pattern I am going to make is rather wide, so I think that the neck and cuff will have to be slightly different from the bottom edge.