A quick and dirty answer might be:

fmap (Data.Text.intercalate ".") resBDwords

I'd probably write it:

Data.Text.intercalate "." <$> resBDwords

but you'd probably want to check your input has exactly two elements. You could use pattern matching:

f (Just [a, b]) = Just $ a <> "." <> b
f _ = Nothing

Then (f resBDwords) gives you what you want I think and avoids head and tail.