There is more than one single locus probe which is used in actual paternity DNA test because we receive half of fathers and half of mothers'. There is more than one probe which is required to test both parents identity.
In other words, locus probes are used to identify multiple intervening repetitive DNA sequence which is located in many chromosomes and results in a generation of 30 and 20 bands, so that we can avoid chances of the same pattern because of multi-band patterns.