def mock_files(mock_dir):
    """Create 2 files: 10MB and 15MB in size"""
    file_name1 = "fake_bam_1.bam"
    file_name2 = "fake_bam_2.bam"

    # Create 10MB file
    f1 = mock_dir.join(file_name1)
    size1 = 10 * 1024 * 1024
    f1.write("a" * size1)

    # Create 15MB file
    f2 = mock_dir.join(file_name2)
    size2 = 15 * 1024 * 1024
    f2.write("b" * size2)

    return file_name1, file_name2