Solved by CasellaJr in post #4 
              
              
                Okay, I found the solution: 
class VGG16SUM(nn.Module):
    
    def __init__(self, model1, model2, num_classes):
        super(VGG16SUM, self).__init__()

        # calculate same padding:
        # (w - k + 2*p)/s + 1 = o
        # => p = (s(o-1) - w + k)/2

        self.block_1 = nn.Sequential(
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