However if you understand that bit at the same time is as you are repeating the first bit as soon as it reaches the same as the shoulder shaping you do the at the same time bit and then either rows 1-3 or 4 (can’t see how many rows it was when typing) as you were previously doing. That’s how I would tackle it
SSK stands for Slip, Slip, Knit which might be misleading. It is a left leaning decrease. First you slip knit wise the two stitches over to the right needle. Then you insert the left needle into both of them by following the tip of the right needle and then you wrap the yarn around the right needle and finish the decrease. It is the same decrease as SKP (Slip, Knit, Pass slipped stitch over). There are plenty of videos on youtube on how to do SSK.
The k2tog is a right leaning decrease while the ssk is a left leaning decrease. If you want the decreases to follow the line of the front shaping, work a ssk on the right front for all neckline decreases and work a k2tog for all neckline decreases on the left front.
(Left front is left as you would wear this very pretty vest.)