Until the advent of the DSM-5, the diagnostic systems excluded diagnosis of Attention-Deficit/Hyperactivity Disorder (AD/HD) and Autism in the same child. However, much debate exists surrounding the validity of this exclusion, with a number of studies identifying individuals who meet the diagnostic criteria for both disorders. The aim of this study was to investigate Electroencephalogram (EEG) coherence differences between two groups of children with AD/HD, where one group exhibited a high level of autistic features (AD/HD+), and the other was low on autistic features (AD/HD-). EEG coherence is a measure of structural or functional connectivity between different cortical regions of the brain. 180 age and sex matched participants, between the ages of 8 and 13 years, were allocated to the AD/HD+ group, an AD/HD- group, or a control group. EEGs were recorded using an eyes-closed resting condition and Fourier transformed to provide coherence estimates in the delta, theta, alpha, and beta bands. The AD/HD- group had significantly elevated theta and beta coherences at short-medium inter-electrode distances intrahemispherically, and across all regions interhemispherically, in comparison to control subjects. Frontal and central/parietal/occipital delta interhemispheric coherences were also reduced. Conversely, in comparison to the AD/HD- group, children in the AD/HD+ group had significantly lower beta coherence at long-inter-electrode distances intrahemispherically, and across all regions interhemispherically. The current results suggest the existence of two independent disorders, distinguishable by their differing beta coherences.