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

It's called an anadrome. Palindrome is a special case where forward and backward generates the same word. Anagram is also related but the letters can be re-arranged in any order.

Examples:

• BATS - STAB
• BID - DIB
• BIRD - DRIB
• BONK - KNOB
• ANIMAL - LAMINA

_{[Examples from The Island English Tutor]}

Semordnilap is apparently a modern alternative. It's "palindromes" in reverse.

Wikipedia source for palindrome and semordnilap

From Macmillan

The word palindrome is an established term in English, used to refer to words or phrases which read the same in either direction. Simple examples are the word noon, or the phrase navy van, which have exactly the same form and meaning when read in reverse. If the word palindromes is itself read in reverse however, the result is semordnilap, a term coined in recent years to refer to words and phrases which make sense when read backwards, but have a different meaning from when they are read forwards.

The British author Michael Quinion seems to have been among the first to give a definition of the term semordnilap, featuring the word in an article for his interesting website, www.worldwidewords.org, in May 2000, though the term is yet to be acknowledged in printed dictionaries. Alternative terms previously used by linguists to refer to the same phenomenon are reversal/reversal pair, inversion and back-word.

(Although, personally, I think Paul S. Lee's answer gives the original word and this one is a fun made-up word because someone didn't realise there already was one!).