Well, in theory you simply take the two pieces with different colours and then sew them together. Because of those spikes I had to do that by hand. Basically every single spike is sewn on its own, then move on to the next one. It's complicated to explain.
Well, before doing that Spitfire all my ponies were completely hand sewn since I didn't own a machine. Now I own one but haven't got much experience so I fall back on hand sewing whenever stuff seems too complicated to do by machine for me.
I thought about just sewing the mango minky on top of the orange one as well, just like you described it. I wasn't sure if I could get it right though. So instead of taking the risk of having to do her mane again I did the hand sewing.