"It is too good to be true."
Translation:Det er for godt til at være sandt.
I suspect it's just one of those things you have to learn by heart. As is so often the case with prepositions, in any language.
Is there a rule for when you used "til at + verb" instead of just "at + verb"?
"At" is just part of the infinite tense as much as "to" does in English: at være->to be, at gå-> to go, at løbe-> to run
In Danish "til" has often the function of "to" in english as they both imply direction, destination, a goal, but not, of course, to form the infinite.
In English when you need to use the infinite after the preposition "to" you use it only once ("I got used to be interrupted", not "I got used to to be a interrupted)
But in Danish there is no such repetition so nothing is omitted.
That's my guess at least, as a non native speaker.