I was kind of looking forward to building my own circuit, if I could find the right plans, or get some help modifying an existing, archived circuit. It would also be cheaper to make a circuit, since I have to buy two $12 timers. I already had two 555 ICs and most of the other circuit components. But if you think it would be too difficult to find existing plans, I'll go with my diagram. I guess I can make enough room for the timers and power supplies in the hood or underneath the vivarium. Do I need any diodes or anything to protect one power supply from power coming back from the other power supply or is the protection already built into them? I've been collecting power supplies for years, and have a couple of drawers full of them. They're the kind that come with electronic equipment, such as keyboards, cordless phones, chargers, etc. I've even got one from the original 1980's Atari video game console. Also, my dual timer won't work the way I thought when I made the diagram. It was a timer I already had, but the instructions were missing and I assumed that the dual outputs could be set independently of each other, but that is not the case. I'll be using two individual Brinks single output, digital timers, instead of the dual output timer in the diagram. If you think my design will work alright, then I'll go with it.
Thanks for the time and advice.