If both Datasets store the same data in the noisy and original format, you might need to use the same indices.
Currently you are storing the first 20 samples in dataset, then overwrite [10:20] with the “second batch” of dataset2.
Probably this would work:
dataset = [dataset1[i] for i in range(20) ]
for i in [10:20]:
    dataset[i] =dataset2[i-10]