Yes, jealousy is likely a big contributing factor to Kiwi's current behavior. He doesn't hate you, but he may be feeling displaced as mentioned above. Keep in mind that our relationships with our birds are often quite confusing for them. Parent? Mate? Rival? flock member? Predator? For the most part, these are the 5 categories that bird relationships fall into in nature. And ofttimes, a bird's person is viewed as their mate. It's the relationship that most resembles their bond with us.
So seeing it from that perspective, it's no wonder Kiwi's feathers got a bit ruffled when a newcomer just strolled (or flew) on in and suddenly grabbed a lot of the attention that was once exclusively reserved for him.
One of the things I did when introducing a new bird was to make a show of giving my attention to him, first. Initially he'll still be less than enthused, of course, but it does ease the transition into a new, flock-oriented perspective. By coming to him first, every time, you maintain for him an aspect of the familiar. He knows when you enter his area, you're coming right over to him. The time is shared, now, but at least he will know what to expect and when. Whatever elements of life before Merri you can maintain will give him a touchstone of the familiar.
Also, you want to institute a definite sense of turn-based interaction. Individual play or training time with one must be immediately followed by individual play or training time with the other. (Unless one or the other is on timeout for bad behavior.) Consistency with this develops a routine, and birds find comfort in routine.
Otherwise, go back to basics with bite pressure training. Some people only correct when a bite becomes painful, but I set my threshold lower. The instant bite pressure leaves the realm of the comfortable, I address it. That way, they don't think their ceiling for bite pressure is at the point where they are causing you pain. Know what I mean?