Not really. Chihuahuas and Great Danes are not, strictly speaking, subspecies. They are different domestic varieties of Canis familiaris and both belong to the same species.
An example of two subspecies interbreeding and producing viable offspring would be, say, the Eastern Kingsnake,
Lampropeltis getulus getulus interbreeding with the California Kingsnake, Lampropeltis getulus californiae and producing viable hybrid offsping, which they can do. As a matter of fact, they have even crossed two different genera to produce viable offspring, like Elaphe (Ratsnakes) and Kingsnakes (Lampropeltis). Given these are not mammals, but reptiles. Nevertheless, the idea that Homo sapiens sapiens and Homo sapiens neanderthalenses (or Homo neanderthalensis, if you prefer)were incapable of interbreeding and producing viable offspring,is, to my mind, not logical.
Chihuahuas and Great Danes are not, strictly speaking, subspecies. They are different domestic varieties of Canis familiaris and both belong to the same species. Darwin described species as "well-marked varieties".
If we populate an an island, teeming with game, with 100 male great Danes and 100 female chihuahuas, and come back in 100 years, there won't be any dogs. Ditto if we use 100 f. Danes and 100 m. chihuahuas. If we use 50 m. Danes, 50 f. Danes, 50 m. and 50 f. chihuahuas, then 100 years later we will find two populations that breed true. (We get exactly the same results using horses instead of Danes and donkeys instead of chihuahuas.)
Therefore, the two breeds of dog must be considered different species in some sense.
Since there is a continuum of breeds that can mate with Danes, with smaller breeds, still smaller ones, etc, all the way to chihuahuas, dogs are an example of a ring species.
I never said whether H. sapiens and H. Neanderthalis were capable of interbreeding or not; no one is really sure, or we wouldn't have articles like this.
BTW, the criterion is producing fertile, not merely viable offspring; think of mules.