Merge Sites

So we have to domains at our company that we have been using. For now I will refer to them as domain 1 and 2. Originally when we started out we only had Domain 1. After some time we improved our system and created Domain 2. We still use some things from Domain 1 but would like to merge it with Domain 2. They are on two different accounts and we would like to merge them under the same domain and account so it is easier to manage and less confusing for our clients. What would be the best way to go about this?