I realize in that design yes, they would be stacked instead of spread across. Semi rigid might outperform the batts, but at 16 inches thick, I think batts will still provide a good broadband effect to the corner modes. You will still bneed to do the other corners, and the rest of the front wall to have it help. Accoustic treatment is best when it is done evenly arround a symetrical room. This creats the balance and reduces the ringing. If you jsut treat the front corners you just hcange the modes in the area and it does strange things to the rest of the room. I doubt if you jsut do those front corners you will get better mixes.
You should also make a panel for the center of the front wall and try and treat the corners of the floor and ceiling at the center , sides, of the mix position. Also treat the rear corners. If the depth is shallow on the room 10-13ft you will need even thicker rear absorbers. There really are no bandaids here. you have to do the entire space or it will just drive you nuts trying to translate. don't forget to make a panel for the ceiling above you or you will be losing your high and high mids due to comb filtering. and 8ft ceiling is a killer without a decent 4"-6" panel above you.