American husband adopting my Canadian children?

Hello all, I have recently been married to a wonderful man who is a wonderful father to my two children. He is not their birth father - birth father has given up all rights to the children and has no access or legal association with them. My husband and I are in the process of obtaining visas for my children and I so that we can become legal american citizens. Until that point, we must remain in Canada and wait for the visa. We have recently come across some security issues regarding my ex husband and his family and due to that, we have been advised by my lawyer that if possible we should have my husband adopt our children. He was unsure about whether or not this would be possible however, seeing as my husband lives in the US and we live in Canada. He is not an adoption attorney and does not have that information. Just wondering if anyone could give us any advice or information, or point us in the right direction. We have been looking online but can't find any concrete answers as to whether or not it's possible for two parents in separate countries to co-adopt their children, or what that would require/entail as far as time lines go. Cost isn't really a factor but we would like it done as soon as possible for the kids' protection. Thank you.