This should work with advanced indexing like this (haven’t tested it yet):
new_tensor = tensor[:, ::2] + tensor[:, 1::2]

Edit: sorry, you would of course have to specify the middle channel like this:
middle_channel = tensor.size(1)//2
new_tensor = tensor[:, :middle_channel] + tensor[:, middle_channel:] 

Sorry for the first part, I misunderstood your goal.