Welcome to the forum, glad you joined.
I think I figured out what that Dr. Higa's is but I'm still confused on the Scent Guard. I'd never heard of either one either. I don't think either one will hurt the chickens.
@JMBern may I ask where you heard of them? I'm just curious. I think they are marketed more for someone using a barrel composter on their back porch than for deep litter.
The Deep Litter Method is effectively turning your coop floor into a compost pile. For the stuff to compost you need certain microbes to eat the stuff. Those microbes are naturally in nature so they will show up, especially if the "compost" is on the ground. But if your first batch is not in contact with the ground then it can help by seeding the right microbes into the compost. I think that's what Dr. Higa's does. I'n not sure about Scent Guard. The way I seed a new compost pile is to toss a shovelful of dirt into it. Those microbes are in the dirt.
For the DLM to work you need to keep your litter at the right moisture level. Those microbes need enough dampness to be able to live and reproduce. If it's too dry they die. If it's too wet the good aerobic microbes die and the anaerobic microbes show up and take over. Those are in nature too and they will show up. The anaerobic microbes are the ones that cause the composting litter to stink. The good aerobic ones give you a nice earthy smell, not unpleasant at all.
If it starts to stink is how you know you have a problem. And it means it is too wet. That excess moisture might come from nature in the form of rain or something else, maybe from a leaking waterer, or their poop. If the poop builds up into a thick mess it doesn't dry out. Poop can build up under the roosts so you often need to manage that.
Again, I don't think either one will hurt the chickens, either separately or used together. I don't think either one will solve a bad smell problem either. You manage that by managing the moisture level.
Good luck and again