 ```python
import os
import moviepy.editor as mpy
from pydub import AudioSegment
import youtube_dl

# Check if video files exist, download if not
video_files = ['video1.mp4', 'video2.mp4']
for video in video_files:
    if not os.path.isfile(video):
        ydl_opts = {
            'outtmpl': video,
            'noplaylist': True,
        }
        with youtube_dl.YoutubeDL(ydl_opts) as ydl:
            try:
                ydl.download(['https://www.youtube.com/watch?v=VideoID'])
            except youtube_dl.DownloadError:
                print(f'Error downloading {video}, please provide a valid video URL.')
                exit(1)

# Load audio, apply fade-in/fade-out, find tempo
audio_file = 'audio.mp3'
audio = AudioSegment.from_file(audio_file)
start_sec = 30
end_sec = 60
audio = audio[start_sec * 1000:end_sec * 1000]
audio = audio.fade_in(500).fade_out(500)
audio_clip = mpy.AudioFileClip(audio.export('temp_audio.wav', format='wav'))
tempo = int(audio_clip.audio.rate)

# Load videos, crop, find looping segment, slow down, mirror
video_clip1 = mpy.VideoFileClip('video1.mp4')
start_frame = 60
end_frame = 120
video_clip1 = video_clip1.subclip(start_frame / video_clip1.fps, end_frame / video_clip1.fps)
video_clip1 = video_clip1.crop(left=100, top=50, width=video_clip1.w - 200, height=video_clip1.h - 100)
loop_segment = video_clip1.speedx(tempo / video_clip1.fps).loop(duration=audio_clip.duration)

video_clip2 = mpy.VideoFileClip('video2.mp4')
video_clip2 = video_clip2.subclip(start_frame / video_clip2.fps, end_frame / video_clip2.fps)
video_clip2 = video_clip2.crop(left=100, top=50, width=video_clip2.w - 200, height=video_clip2.h - 100)
mirrored_segment = loop_segment.fl_time(lambda t: video_clip2.duration - t)

# Combine videos, add audio, create title/credits screens
final_clip = mpy.CompositeVideoClip([loop_segment, mirrored_segment.set_position((loop_segment.w + 5, 0))])
final_clip = final_clip.set_audio(audio_clip).audio.fade_in(500).fade_out(500)

title_screen = mpy.TextClip('Title Screen', fontsize=32, color='white')
final_clip = mpy.concatenate([title_screen.set_duration(2), final_clip])

credits_screen = mpy.TextClip('Credits Screen', fontsize=32, color='white').on_color((0, 0, 0))
final_clip = mpy.concatenate([final_clip, credits_screen.set_duration(3)])

# Write final video to file
final_clip.write_videofile('final_video.mp4', fps=25, audio_bitrate='192k', bitrate='10000k')
```