The causes underlying comorbid learning difficulties in reading (RD) and math (MD) are still a matter of debate. Based on current research, two models for the relation of the cognitive profile of isolated and combined learning difficulties (RDMD) are discussed. Regarding the "multi-deficit model", the profile of RDMD is characterized by the sum of domain-specific core deficits of RD and MD ("additivity") as well as shared domain-general risk factors of RD and MD resulting in less severe deficits than expected under additivity ("under-additivity"). The "three independent disorders model" explains RDMD as a distinct learning disorder, showing a separate cognitive profile with distinct and/or more severe deficits, compared to the sum of RD's and MD's profiles ("over-additivity"). To evaluate these approaches, a meta-analysis including 74 studies, examining children aged 6-12, was conducted. Separate group comparisons for the three subcomponents in the cognitive profiles--reading, math, executive functions (EF)--were considered. Linear hypothesis testing revealed different results regarding the three subcomponents of the cognitive profiles of children with isolated vs. combined learning difficulties: Whereas RDMDs' deficits in reading and math represented the sum of the deficits in the isolated groups (additivity), there was some evidence that RDMDs' deficits in EF skills corresponded to under-additivity. Furthermore, group differences in math skills were more pronounced in symbolic than in non-symbolic math tasks, whereas in reading, group differences were larger in phonological processing and reading than in rapid automatized naming and language skills. Results are discussed in terms of intervention options for RDMD.