That would depend on the type of fleece you use. If it's malden mills 300 then you shouldn't as it is good enough on it's own. The thinner fleeces usually don't work with one layer so they need two.
The other thing is the type of pattern you are using. I have a swaddlebees cover made from a single layer of fleece and it's got an elastic casing sewn onto it and TBH it looks a lot more complicated that the ones that are made from two layers.