You can be infected with same genotype subtype such as 1a & 1b but you can't be infected with two distinct genotypes such as 1 & 3 or 1 and 2.
If you are exposed to a different genotype the existing genotype will become the dominant genotype and eradicate the newly introduced genotype is my understanding.
Richard Sallie's 'Replicative Homeostasis III' and the extensive references section at the end of his paper might suggest some of the reasons for this.
A little light reading for sleepness nights.
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If I try explaining it, he might start posting here again, if only to correct my bungled explanations ;-).