You won’t be able to create the output as [64, 100, 18, 18], since this would double the number of all available elements.
However, you could create a tensor in the shape [64, 100, 18, 9] or [64, 100, 9, 18] via:
x = torch.randn(64, 100, 9, 9)
y = torch.randn(64, 100, 9, 9)

print(torch.cat((x, y), dim=2).shape)
print(torch.cat((x, y), dim=3).shape)