It is known that a backward Schramm--Loewner evolution (SLE) is coupled with a free boundary Gaussian free field (GFF) with boundary perturbation to give conformal welding of quantum surfaces. Motivated by a generalization of conformal welding for quantum surfaces with multiple marked boundary points, we propose a notion of multiple backward SLE. To this aim, we investigate the commutation relation between two backward Loewner chains, and consequently, we find that the driving process of each backward Loewner chain has to have a drift term given by logarithmic derivative of a partition function, which is determined by a system of Belavin--Polyakov--Zamolodchikov-like equations so that these Loewner chains are commutative. After this observation, we define a multiple backward SLE as a tuple of mutually commutative backward Loewner chains. It immediately follows that each backward Loewner chain in a multiple backward SLE is obtained as a Girsanov transform of a backward SLE. We also discuss coupling of a multiple backward SLE with a GFF with boundary perturbation and find that a partition function and a boundary perturbation are uniquely determined so that they are coupled with each other.