I’m not sure what your input sizes are, but you do have a couple of choices:

cat along channel dimension
stack along H or W dimensions

For example, if your images have size (300, 500) and are tensors of size (1, 3, 300, 500) (this is NCHW), you could do torch.cat([tensor1, tensor2], 1) to cat along the channel dimension.
You could also do torch.stack([tensor1, tensor2], 3) to create a new tensor of size (1, 3, 600, 500) that is literally the two images side by side.