Please please make sure you have oxygen on board and a very good first aid kit. Bare in mind that if both of you are in the water together, it means you will both have the same dive profile. All the more reason to dive very conservativly, if something happens to both of you there is nobody on the boat to help you. Perhaps set up a system where you radio and let someone know your dive plan and that you will radio back upon ending the dive. It can be done but ensure that you both have the correct rescue and emergency training, a survival pack (the one mentioned earlier was excellent) and that you really are sure of what you are doing. Normally it is the divemaster's job to organise emergency plans and take care of the emergency equipment, this now becomes your own responsibility.
Safe Diving
Safe Diving