Solved by ptrblck in post #6 
              
              
                Sorry, your previous explanation would work in this case: 
a = torch.randn([2, 1, 16])
b = torch.randn([2, 1, 64000])
c = torch.cat((a, b), dim=-1)

as I’ve mistakenly saw torch.cat((A, B), dim=1) in your initial post instead of dim=-1.