Learn English – What do you call a word that can be spelled forward or backward forming two different words

The closest I believe is "cognate" (from Merriam-Webster Online ):
3
a: related by descent from the same ancestral language
b: of a word or morpheme : related by derivation, borrowing, or descent

However, translators must beware of cognates, because the meaning and usage often varies in subtle ways between languages. For instance, "mist" in German" is what farmers put on the field at night, causing a steam (ie a 'mist') to appear above the field the next morning that obscures sight.

This is a question that is discussed by eminent British linguist David Crystal in his article How many words?, in which he attempts to come up with a reasonably accurate count of the number of words in the English language. Basically, he says that no accurate number is possible because there is no universally-agreed definition of what constitutes a word. Here is an extract that addresses your question, but provides no definitive answer:

Is the lock on a door the same basic meaning as the lock on a canal? Should ring (the shape) be kept separate from ring (the sound)? Are such cases 'the same word with different meanings' or 'different words'? These are the daily decisions that any word-counter (or dictionary compiler) must make.

The article is here (pdf): www.davidcrystal.com/?fileid=-4890