http://www.mangago.me/read-manga/doutei_doudeshou/
And in the sequel they switch
http://www.mangago.me/read-manga/doutei_doudeshou_dj_another/
Look at this. You'll be surprised how many of Seke.
http://www.mangago.me/tag/Seke/
What does seke mean?
seke = seme uke. both have the position to be either seme or uke, so the roles are reversible. in other words, one can be bottom and then the next time they have sex that person can be top.
Seke couple mangas?! (They are so rare, man lol) I've already read Ogeretsu's Reversible