set your second character up to use char1 as a focus
/focus char1


then you have your macro on char2 (i key)

/castsequence [target=focustarget] curse of agony, corruption, curse of agony. Thats if you want to use dots otherwise just sub out them for shadowbolts

By adding this on your second char, it will follow your main character as the focus and will use char1's target as your alt's target. Heaps of stuff in the stickies for more info.