J.L.
Yikes, I am so sorry for you. This is going to be tough no matter how you do it, but I really don't think you will traumatize your child by weaning him and moving him to a separate bed, even if you do it simultaneously. I agree that moving first will make it easier to wean-but I would argue that regardless they go hand in hand. Granted, it won't be easy and possibly would've been easier if done earlier, but I can't imagine making my husband sleep on the floor for twenty months. He deserves a medal for most patient spouse alive.
At 20 months there is no physical need for your child to nurse at night (ask your pediatrician to be certain, but most babies can sleep through the night without nursing by 8 months- and often earlier) and continuing to do so will end up allowing poor sleeping habits to develop that the longer you let go the harder they will be to undo.
Whatever your method or decision you need to remember 2 things 1. Be consistent-if you commit to weaning and changing sleeping arrangements, be prepared to follow through-no matter what. Your son will adjust, but it will be traumatic and confusing if the rules keep changing and one night he can sleep with you and one night he can't etc. I think making a big deal out his own big boy place to sleep is a great idea.
2. Remember why you are doing what you are doing-your marriage is important-without respect, unity and stability between you and your spouse, you will send the message to your child that HE is the most important equation in the family ( of course in a way he is, but he can't learn that His desires rule that family, he must learn to trust that Mom and Dad to make healthy decisions for him). Also, you are doing this for his own good. he can't nurse forever, he can't sleep in your bed forever and it is healthy and important for him to learn to comfort himself and learn to sleep through the night so that he develops healthy, normal sleeping habits.
Weaning a child and expecting them to respect Mom and Dad's space is not mean. You are obviously trying to what is best for him and your family. Personally, I really feel it is important to respect my spouse, so even if I would prefer a gentler approach I would honor his wishes if after we've discussed the matter it still seems like a feasible plan.
another thing to remember is that during child development children do not remember every single event that happened to them- I don't remember being weaned, etc. I know some people argue that every experience will affect your child and that may be true-I am sure it is partially true- but what you are doing is good for him-good for you and good for your marriage, so I think in the end it will affect him positively. The resistance you'll get is going to be uncomfortable, but it is normal for children at every age to resist parental decisions that they don't like. It is ok that he will be upset-remember you are doing the right thing. He will recover and he will still love you and your marriage will grow stronger. Be encouraged-it won't be easy but it will be for the best.