Tsubasa: Tokyo Revelations

The whole Tokyo Revelations OVA is based off of X, it has the same characters, setting and overall dark nature. Both series are by CLAMP. 

Code Geass and Tsubasa: Tokyo Revelations are similar in someway whereas the main setting Japan which Tokyo is part of is in a desperate state (Japan is colonized in Code Geass and Tokyo is in ruins in Tsubasa: Tokyo Revelations) as groups such as the Black Knights and Kamui's and Fuma's respective parties search for a way to restore their homes back to normal. Lelouch's character design is similar to Kamui and Suzaku's to Syaoran due to Clamp doing both the character designs. Twists and betrayal occurs as well as revelations that will change the story forever. Both shows have dark tone and action packed fights as well though they are different in terms of style (Code Geass-Mecha and Tsubasa-Magic). Try watching one of these if you've watched one of them which will be a great addition to your collection! read more 

Both invoves a complex love story that is the fuel to the rest of the story, where the love is so big that it lead the whole universe to bigger events. 

Both show's setting (Japan and Tokyo) are in ruins at a post apocalyptic state especially finding a way to cure the disturbance that affected it. Both have dark tone, twists, powers, revelations, interactions, and romance.  

Characters set out in a different world involving canonical materials of the main story.  

Both series are a crossovers from different series with the main male lead chasing after the female lead with a strong cast of characters. Also the XYZ Dimension arc and the Tokyo arc are identical in terms of setting and plot. 

Both have kind of post-apocalyptic world with strange monsters. And a brave princess with a pet (Mokona and Teto). Both of them loose someone important to them.  

Although the storyline is different, the same characters are present and are considerably younger. This series is fun, but doesn't contain the same action-packed adventures as in Tsubasa Chronicle, andconsists of a more 'mild' type of action. However, in the Tsubasa OVA's there are some hints pertaining to this anime, which simply adds to the spice of it all and makes you wonder how linking both anime's together adds to the storyline. Overall this is a fun and enjoyable series and I suggest that all were to watch it. If you're a fan of Sakura, Syaoran, Tomoyo, and the rest of the old gang, you'll find this story to be adventurous all on its own. Enjoy! read more 

Both series are set in a fantasy world. Both series incorporate a quest that the main characters are trying to achieve. The two series may have one universal prerogative but for the most part they are episodic in nature that try to connect with the main objective. Both series are laudatory and reverrential due to their fantastical world setting. 

Both involve lost memories and groups on an adventure. Both stories turn into a serious drama. And they involve romance. 

Both series involves time and dimension travel. Also, the male protagonist in both series travels and battles against his other self to save the girl that he loves. 

It's completely different in every way but one - they do not fit in to the series story-line. It's just random extra that you don't know what to do with. In "Tokyo Revelations" case it's a part of the true story-line from the manga which doesn't fit into the fluffy story-line of the anime. They're both worth a watch ("Tokyo Revelations" more so) but when you go back to the real series you have to forget everything that has happen. They're both good places for spoilers as well so I recommend watch after you've seen and/or read everything~ else 0:-) 

well, Same authors and apper of some characters of the move and if you pay atention you will see in the end of x/1999 the beginning of tokyo revelations. this is for Tokyo in ruins. 

If you watch episode 11 or 12 -- can't remember...--- of xxxHoLic Kei, you'll see that the Watanuki's favor for the Cat-spirit affects the people in the Tsubasa Chronicle Tokyo Revelations. I won't reveal any further for spoiler reasons....