Wortwörtlich translates as “word for word”
Tagtäglich as “day by day”
Are there any other examples of this formation? Does it have any rationale (etymological or otherwise)?
Yes, there are, but they are not very common
You even can consider them extremely rare. Rather to be found in poetry (i.e. creative use of language), not ordinary utilitarian texts.
The phenomenon is called Reduplikation or Iteration, and you have chosen the two most prominent and probably the only two accepted as "real word" examples for this in the German language. The words are built this way to sound interesting, are easier to understand (forbabies) or amplify the meaning. Sometimes they also are loanwords, made to sound ridiculous.
If you accept Children's language, and even slight variations to the duplications, here's an absolutely non-exhaustive list of some other more or less exact reduplications (interesting how some of these are present or lead to other reduplicated words in foreign languages)
grundgütig and tieftraurig are maybe no reduplications but follow a very similar pattern.
A similar doubling in the word formation as in wortwörtlich (word-for-word) or tagtäglich (day-to-day) we might also want to consider for the technical term eineindeutig (one-to-one) as in
eine eineindeutige Abbildung (a representation having a one-to-one correspondence with its original)