Why do we need two markers to measure a recombination rate?
If at least one locus is homozygous

If a recombination event happens between two loci where at least one is homozygous, then you would not see anything. Consider for example the following strand sequences in a diploid individual

-----A-----B----
-----a-----B----


Whether a recombination event occurs or not, the two possible chromosomes passed down to an offspring are 

-----A-----B----
-----a-----B----


Therefore, you cannot tell whether recombination has occurred.

If both loci are heterozygous

Now consider the following individual

-----A-----B----
-----a-----b----


If no recombination event has occurred between the two loci of interest then the two possible chromosomes that will be passed down are

-----A-----B----
-----a-----b----


If on the other hand a recombination event has occurred between the two loci of interest then the two possible chromosomes that will be passed down are

-----A-----b----
-----a-----B----


Therefore, you can tell whether a recombination has occurred or not.