"Das sind zwei Geräte."
Perhaps it should be are, but we would never say that in English. We would say "That is two devices." Look at it this way: the set of devices is seen as one entity, similar to how ein Paar is one entity in German, and yet in English we would consider a pair plural... sort of weird that we flipped on this one. That is two devices is grammatically correct because it is referring to the single entity that would contain all those devices (replace "two" with "set" to get this: That is a set of devices; notice a set, implying only one set that contains two devices). If we were asserting them separately, we would state "Those are two devices." Either sentence would describe the same thing. Typically though I would say English speakers would probably say That is two devices if the whole grouping was two, and we would say those are two (of the) devices to describe a subset of the larger set. I hope that makes sense.
It should never be "That are", 'that' is a purely singular demonstrative pronoun. It is different in German AFAIK, which means 'Das' can mean that, this, these or those. The fact that the German sentence has 'sind', you should be translating this as "These are two devices" or "Those are two devices".
Don't confuse Das with Diese/Dieser/Dieses, both can mean either this or that, but Das is a pronoun and Diese/Dieser/Dieses are demonstrative articles.
Correct me if I'm wrong: It is possible to use das for more than one object. In English, however, the plural of 'this' is 'these' (and 'that' is 'those' which I suppose would be accepted as well). Dieser, diese, dieses (depending on gender) are also demonstratives, but they are not pronouns (please confirm!) and require to be followed by the noun (as 'this car'). In plural it would be diese for all genders, with the same restriction.