Could you put a baby gate on the door of the bathroom and let him play with some toys in the bathroom while you shower? Put locks on the cupboards and such to babyproof and keep a special basket of "shower time" only toys?
When I had just one DS, I would actually put him in his high chair with a sippy cup to watch a cartoon while I showered. Our house is set up where I could set his high chair right outside the bathroom door so i can hear him the entire time and see him if I pull the curtain back.
A few times I actually even resorted to showering with the curtain completely open bc I was so desperate. Lol. That was a day when our computer wasn't working (where we watch cartoons) and after we'd gotten rid of the high chair.