We consider adaptive two-channel multiple-description coding. We provide an analytical method for designing a resolution-constrained symmetrical multiple-description coder that uses an index assignment matrix. We use existing index assignment algorithms that are known for their good properties within our adaptive multiple-description coding architecture. These existing index assignment algorithms are parameterized and the coefficient of quantization of the side coders is described by a rational function. This leads to an analytical solution for the design problem, facilitating real-time adaptation based on information provided by feedback channels or rate conditions imposed by network management. Our experimental results show that the practical performance closely approximates theoretically obtained optimal behavior.