This is something that I have observed and pondered on. while I haven't researched it very heavily one thing is true. Those two names have only one Japanese character and a single syllable difference.
Sagara Sousuke is the name of an over militant character in the series Full Metal Panic! who uses military action no matter where he is even in a civilian environment.
Sagara Sanosuke is a hardened fist fighter in the show Rurouni Kenshin. He loves to gamble and likes to solve things with his fists.
You may say "ok so what?" well the thing that makes this interesting is the following tings. First look at Sousuke's face he has a cross scar on his left. Second FMP was made after Rurouni Kenshin. Now look at the character Kenshin who Sanosuke is co-star to.
Himura Kenshin the main character in the show Rurouni Kenshin. A man with a violent past and leading a now rather docile life.
The observation here is that Kenshin has a cross scar on the same side of his face. The thing I'm seeing is obvious though I have no proof. The man who did FMP was a fan at least of Rurouni Kenshin because of the uncanny likeness of Sousuke to two major characters in the Rurouni Kenshin series. It's like he's a combination of the two with a very slightly altered name. Like I said no proof but in my mind it's almost for sure that there is some definite link between the two even if it is as small as fandom.
Coincidence? probably not.