Supplementary vote is a voting system for selecting one person. Each voter may make a first and second choice (no more!). The top two candidates, on first choice votes, are retained. Where a voter voted for a candidate other than the top two, their second choice vote is applied, if it is for one of the top two candidates. This system is used for Mayoral races in the UK. SV is viewed as encouraging a more positive style of campaigning as candidates desire the second preferences of third parties.
Two-choice party voting has some similarities, except that it is used to measure voter support for parties under MMP. Parties passing a set threshold, on first choice votes, continue to the next stage, where they receive the second choice votes from voters whose first choice party did not pass the threshold.
Both of these systems are based on the idea that first and second choices are often clear in the voters minds. Using lower level preferences is less useful and makes complex many aspects of voting and vote counting.
These systems are simple, but get most of the benefits offfering preferences, while avoiding the complexity of multi-level preferences. Two-choice party voting means every voter can both vote for whichever party they wish, and have a say in the allocation of seats in parliament.