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There is no need for two markers, one in each flanking region.
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I disagree on this. Here is why.
In order to select for a unique gene, you must have a marker that is near that unique trait and only exists in one of the breeding lines. It does not matter what the other line has at that location, all that matters is that one line or the other is unique. Then you have to consider the other end of the gene. If you are introgressing a trait and a huge segment of a chromosome crosses over, then you are going to be stuck with whatever genetic contribution that chromosome segment includes. Therefore I reiterate that in order to introgress a gene, you MUST have 2 markers, one on each side of the gene. Otherwise linkage drag will make your breeding efforts useless.
More importantly, with some traits where crossovers are rare, you might want to use 4 or even 6 markers so you can progressively narrow down the trait.
DarJones