You must never, ever, land on an occupied runway....period.
Err....
, I'm afraid
From UK CAP413 R/T manual, but similar to other places around the wold:
A landing aircraft may be permitted to touch down before a preceding landing aircraft has vacated the runway provided that:
1. the runway is long enough to allow safe separation between the
two aircraft and there is no evidence to indicate that braking may be
adversely affected;
2. it is during daylight hours;
3. the preceding landing aircraft is not required to backtrack in order to
vacate the runway;
4. the controller is satisfied that the landing aircraft will be able to see
the preceding aircraft which has landed, clearly and continuously,
until it has vacated the runway; and
5. the pilot of the following aircraft is warned. (Responsibility for
ensuring adequate separation rests with the pilot of the following
aircraft.
As used at such minor airports as Heathrow & Gatwick.