Dim 0 means that you treat the outputs as independent batch samples since you increase the batchsize. However, this is probably not what one would want in this case,since the results of both models belong to the same sample.
Using dim 1 concatenates in each samples channels which makes more sense since they are now belonging to the same sample and this approach can (unlike the first one) even be used if the model’s produce a different number of channels (although the rest of the sizes must be equal)