I can not find a way to have both a natural and a flat accidental in front of a single note.
How can I get this in LilyPond?
This might be what you are looking for:
\tweak Accidental.restore-first ##t
cis!
after a cisis
in the previous measure.
As Ramillies says in the comments, this is a pretty rare occurrence; I'd be curious to see the context of your example. But the main reason I can think of these accidentals happening is to cancel out a prior double accidental (with the natural) while still giving a single accidental to the new pitch.
But LilyPond has various accidental styles that automates how accidentals appear in your scores. Depending on your specific example, you may find that one of these styles fits your needs the best.
beses4 bes4
, the first note will have a double flat and the next one a natural + flat. Outside of these cases, such a notation would be extremely rare or nonexistent in actual use.