Depending on the interpretation of combination of {gerku} and {si'e}, it can mean heads or tails of dogs, but I don't think that interpretation reflects a usual meaning of {gerku} and {si'e}.
{pi PA SUMTI} is defined as {lo pi PA si'e be pa me SUMTI}. According to usual meaning of {si'e}, the referant of x1 of {pi PA si'e} should have the same property as the referent of x2 of {pi PA si'e}, though that interpretation is not officially declared. As shown in the example {pi re lo mu gerku}, some selbrisni like {gerku} do not fit the usual meaning of {si'e} because a part of dog is not a dog in usual meaning.
In the case of {pi re loi mu gerku}, which is developed as {lo pi re si'e be pa me lo gunma be lo mu gerku}, x2 of {pi re si'e} refers to a collective group of the referent of {gerku} in the meaning of xorlo, and then the referent of x2 has the property of {gerku}. Therefore, according to the usual meaning of {si'e}, x1 of {pi re si'e} should have the property of {gerku}. However, according to the usual meaning of {gerku}, a head or a tail of {gerku} does not have the property of {gerku}. In order to satisfy the usual meaning of both, x1 of {pi re si'e} should be one dog.