Answer
Two hypotheses: 1) The two mushrooms are produced from the same mycelium which has spread across the yard by mitosis and growth; 2) The two mushrooms come from two mycelia which grew from two identical spores produced from the asexual part of the fungal life cycle.
Work Step by Step
To get this, look at the fungal lifecycle and ask yourself: when are cells genetically identical? This will be when mitosis has produced them, and any mushrooms coming from those cells will also be identical.
