As others have mentioned, you don't need a cert for doubles .... but don't tell PADI for gawd's sake.
I have two sets of doubles, one smaller than the other (AL50 x 2 and HP7.9L x 2) both are very manageable for me weight wise, although the HP set is a PITA when walking over soft sand at the beach.
I talked to a few people before having them set up, of course this necessitated in moving to a wing set up, which I used with a single tank for about 20 dives first. Too many new things on a dive is not a good idea.
I played around off the beach in about 3 meters of water (I have access to a pool but it is used a lot by little kids who I am sure pee in it and I have had enough issues with ear infections in the past so I don't use the pool). A friend of mine buddied with me to make sure I did not run into any trouble. Initially I had some issues with trim, too much weight up top because of the extra weight on the manifold, add a housed SLR and I could see me diving upside down, however with the use of ankle weights (5mm suit) it worked out on my first dive, which I ran with the manifold closed allowing me to have redundancy.
After a couple of dives with the manifold closed I moved on to diving with the manifold open and I started practicing shut downs and closing valves, something you can also do on dry land but it is actually easier in the water.
I only use my doubles for some specific dive sites, and occasionally do two dives with my HP set, saves changing tanks during the SI when everyone else is flaffing around.
I might do a tech course one day