I can’t have been the first to think of this, but I found this interesting the more I thought about their individual characters. (AND NO THESE ARE NOT SHIPS)
 
Sahnic Rainboom: Both fast (duh), possesing an easy-going demeanor and somewhat short temper, yet strong sense of justice, compassion, and love for freedom and adventure. Both fiercely loyal to their friends.
 
KnuckleJack: Both supernaturally strong, hard-headed, big-hearted, and both headstrong and serious, yet honest and short-tempered to a fault.
 
FlutterCream: Both shy, good with animals (or chao), polite and curious, but assertive when necessary and very kind.
 
PIIIIIIIIIIIINK!!!: Both fun-loving, cheerful, happy, and hyper, but hurt someone close to them or break a Pinkie Promise and you’d best be runnin’ son.
 
Twilight Blaze: This is a fun one. Both princesses, both guardians of the prime power of their respective worlds (Sol Emeralds/Elements of Harmony), level-headed, a natural leader, and extremely magically gifted (pyrokinesis = magic). Also both were “initially unfamiliar to the concept of having friends, but after spending time with Sonic/Mane 6 [they] discovered the true meaning of friendship allowing [them] to accomplish tasks for her people which she could never do alone.” -Straight from the Sonic Wiki. (Also Twilight Blaze is really a really cool name imo.)
 
Assault and Bat-tery: Both Jewel hunters, very talented in martial arts, both obsessed with glamour and naturally beautiful. Both use their charm on males to get them to do what they want. Both are extremely clever, even though they don’t always appear that way. “Once she has a target in mind, she goes for it regardless of the personal cost” which could be construed as generosity I guess. (Gimme a break.) - Sonic wiki again.
 
Thanks for reading!
 
MLP owned by Hasbro
 
Sonic and other characters owned by SEGA (And Nintendo, but yeah.)