Mujaki no Rakuen

Adult protagonist with unsatisfying life gets send back in time and ends up in his childhood in his body as a child. One chooses to take this opprotunity to endgage in ecchi escapades while the other one tries to solve a murder mystery. Both are seinen. 

They both share a similar theme and are each very lewd in nature. 

Both are ecchi comedy set in elementary school, with short episodic situations among students (there's no adult character in either show). The humor is similar, both create risque situations out of either groups of boys vs. girls pranks, or by one on one pairing. The main difference is that Kyou no 5 no 2 remains fairly tame, stopping at PG-13 grade panty shots and under-boobs. It's comedy, mainly slapstick, facial distortion and violent retribution by the girls, remains in the driver seat. In contrast, Mujaki no Rakuen goes the extra mile to borderline hentai and is quite exploitative, lewd and explicit at it. To make this even more questionable, Rakuen's MC is an adult trapped in the body of a boy. read more 

Unsuccessfull guy is sent back in time to his child era and proceeds to do lewd stuff. 

Lucky perverts. Always getting into hijinks with the opposite sex whether they like it or not.  

Adolescent high jinks with a similar "girls run the show, boys watch out" feel. Water guns and pudding. Obviously, Mujaki no Rakuen is more sexual.