S.S.
I don't think it's _just_ testing boundaries. I have a 21 month old who is very strongly attached and has gone through periods of intense separation anxiety. Every child is different, but this is very common. It is episodic as the child becomes more aware of her surroundings and ability to move independently (i.e., it's fun and exciting to move away, but don't want to get too far away). When my son was 17 m, I was 8 months pregnant and so my husband had to do a lot more care-taking. My son seemed to regress a lot. He was resentful and would push his daddy away, say "no daddy" and so on. He would even take the bottle away from daddy and hand it to me, to make me feed him. It also seemed to me that he was blaming daddy for my absence. However I had little choice since I was so exhausted and knew I would need to let go even more once the new baby arrived. We never ferberized him, although sometimes I needed to just leave and let the two of them work it out. The two main things were that my son didn't want anyone else to feed him or put him to sleep. So there were a few times when I was there, but feeling sick and couldn't feed him. So he cried for half an hour - which is a long time! We both offered to hold him during this, and my husband kept offering him food while I stayed in bed. He eventually ate. But sometimes when he asked I also fed him. We were not absolutely consistent about it and he still grew out of it. My husband was amazingly gentle and compassionate with him, realizing that it was not that our son hated him, he just wanted his mommy. So tell your husband not to take it personally. As for the sleeping thing, that was harder, but I had to just let my husband do it and not intervene. Now he puts him to bed every night, while I put the baby to bed. Occasionally I'll put him to bed, but he seems to do better with a regular routine so I don't do it too often.
For us at least, part of the issue is habit - what he expects to happen. If mommy puts him to bed every night for his whole life, of course he feels freaked out if someone else tries to do it. In a more general sense, if mommy has always been the one to meet his needs, he needs to learn that someone else can also do it, and this takes time. It doesn't mean either of you are bad parents or there is anything wrong with your child. There is just a lot for a little toddler to learn!