regardless of voice or not you need to initially separate them to train any behavior successfully. it will be easier, faster, and stick better for both dogs. I do separate voice and hand signal only training sessions for my dogs so by all means hand signal only sessions are perfect for you. if you don't already get some clickers and leave them all around so that you have one when needed. i generally have a pocket of treats or a string cheese stick and a clicker in my pocket most of the time anyway. freeze dried meats or can cheese also make easy, portable, neat, treats.
the nice thing about clickers for you is that clicker purists believe that when you talk you actually could be polluting the clarity of click/treat by distracting from that singular moment in time that i marked with the clicker. the click/treat is the praise so no verbal is needed. i do a lot of talking to my dogs out of habit since were together all day so it's not a hard requirement but it may be a distinct advantage to have no verbal input.
I got both dogs to the point of knowing commands separately and then i brought them together and worked on doing group sessions. for these have an all dogs cue i say "puppies sit" or point to both meaning both sit. you might point at both dogs one with each hand as an all dogs do this next command. i also suggest training all hand signals with both hands so that the dogs are fluent and understand better that it is a signal or cue not a very rigid body behavior on your part. this way too if you give off unconscious non verbal cues like bending or cocking your head the dog will isolate more on the intended signal which is the hand. be conscious of what you are doing with the rest of your body. try to stand like a statue (relaxed posture important) but don't move your head, bend, etc. your cue hand should be still when not cueing or behind your back and the clicker hand should also be still. hold the food and clicker in one hand behind your back or leave the treats on the cabinet out of sight. the reason for hands still is you don't want then interpreting other stimuli as cues like rustling a bait bag or plastic.
for the individual work crate one or leave one inside with a great chewie or kong (but not too much or he won't be hungry for training). you need to come up with a hand signal or other noise that each dog associates with themselves like a name. it could be clap or foot thump loud enough to be heard from another room. or you could use that as a come and then give each dog a "name" or signal that is theirs. whatever works best for you logistically.
once each one learns their sign which you can teach by playing the name game the same as an audio name. cue name, dog looks, click treat. when both dogs are solid on their name and the same few tricks bring them together again but for designated name cue training. you can use the dog's "name" and/or teach the dogs I point at you and you do the command and you and only you get click treat. whichever is simpler for you and the dogs will work. if the other dog responds to this dogs "name" ignore the non-called dog. Finish up with the first dog then you can cue the other and give the chance to understand this is for you now and the other dog is not included for this brief moment. you can see that some basics like down stay would be valuable for this group session so you can isolate dog a and dog b.
I have a different clicker for both of my dogs though it wouldn't be necessary. the digital clicker makes pinging and other sounds that i used initially to teach my older dog calming lessons and a regular clicker was stimulating hence counterproductive. the puppy never gets treats for a ping so it's meaningless to her. the older dog does know the regular clicker though so i need to be very specific about who i am asking and who gets treats. i ignore puppy if she offers a paw when i ask the older dog. but then i turn to puppy after a moment and cue her to do a different behavior and click treat her then ask for a paw and click treat. so that she's not discouraged but motivated to think and given another opportunity to earn the treat quickly. the downside of digital clickers is they break pretty easily-I’ve gone through 6 in a year and they have batteries to replace. they are also about $15 compared to 50 cents for a box clicker. so you can decide if it's worth it.
there are a number of sites, forums, and groups for teaching deaf dogs that would also be very helpful for you. All of the things there would apply to a voice free training environment. such things as wireless strobes or bells with remotes, vibrating collars, and other equipment could also help you call your dogs when they might be out of hearing range.
recommended reading and links
Living with a deaf dog by susan cope-becker seems to be highly regarded and anything from karen pryor's site on clicker training is great and there are state listings of KP certified trainers. I love my KP trainer. www.clickertraining.com
Good luck and I hope this helps. Many days I can't depend on my voice so the hand signals make a huge difference. Let me know if this helps or is sort of what you needed. If you want more help with the issue of Magic's shyness. My 4yo is also shy but recovering nicely after a setback but was nearly normal for 3 years. It can be helped with training and with a trauma like a dog attack set back but training will work.