Like I mentioned elsewhere, I'm not too fond of the whole Yuna/Lenne and Tidus/Shuyin cameo theory. However, there are some links between them that we cannot overlook as mere coincidences.
Name
Literal translation aside, Yuna and Shuyin can easily pass for native Japanese names while the same cannot be said for Tidus and Lenne. Assuming we're on the same page as creators of FFX/X-2 -- some sort of Yin and Yang theory could be at work here.
Sun Calendar is used in the west to the contrary of Asia's Lunar Calendar. In Yin Yang philosophy, sun is symbolic of Yang and moon is symbolic of Yin. If Yuna and Shuyin originated from the east (aka. Yin), Tidus and Lenne could be children of Yang judging by their west-oriented accents. Likewise, a balanced relationship requires equal portions of Yin and Yang to take place; this actually fits the role given to our characters: Tidus was the "sun" that brought light and hope into Yuna's doomed fate as a summoner, Lenne was the "sun" that soothed the darkness within Shuyin's heart.
Model
There was once a popular saying in the fandom that Tidus could have been a dream image of Shuyin. Validity aside, said theory reminds me of the series of events that triggered Shuyin and Tidus' respective ends, why they were so different despite similar life situations, vice versa. Overall, Tidus was the "sun" in every aspect of his relationship: although he worshipped Yuna as much as Shuyin adored Lenne, he cared for the well-being of Yuna's people just as much. In other words, Tidus was more perceptive of Yuna's character than Shuyin was ever of Lenne's: how could someone with Yuna and Lenne's sense of duty be truly happy without Spira in good shape first?
I believe there to be a hidden motive behind the portrayal of Shuyin/Lenne's relationship. In East Asia, the role of Yang is usually exclusive to males. In Shuyin and Lenne's case, however, we see a distortion to said trend. This could be why their relationship met with a tragic end: the natural flow of man-woman relationship was accidentally bent. Tidus and Yuna's happily-ever-after was probably meant to amend that, especially if Tidus was in fact created based on Shuyin's character.