My husband and I play split screen. We don’t always choose characters to play as a combo but when we do we are:
Miko+Galilea: Great pocket healing. Not as easy as it looks but once you get into the motion of things you will be fine. Galilea needs to pick off enemies individually if possible and Miko needs to cover the rear for heals. Watch out because people will or should attack the healer ie Miko. Galilea can hold her own against some characters so use Miko for group healing as well but always communicate with your gf so you can save her when needed. If you’re playing as Miko. Yall can rotate or play whichever roles yall prefer 
Miko+Montana: Same miko strat as aforementioned and use Montana as others have suggested. Ice build. I personally don’t choose the wind up every match so whatever works for you guys. The dash we use offensively and as a getaway. Depending on your situation and the opposing team you will need to utilize skills differently.
Really, Miko+Anyone is good 
Caldarius+Rath: I play Caldarius a lot cuz he’s super fun. When in PvP he’s an okay match with any melee but best with Rath. Y’all can both get in and out with ease. Caldarius is very mobile and you need to keep him mobile, like with any ranged you want to bounce around. With Rath you can go for the silence, knock up, and atk atk atk with both Rath and Caldarius. Simple and basic tactic but customizable. With Caldarius you can use his SMG but I utilize his blade about 50/50 once you get lifesteal and atk speed. His dash is the same as Montana in the sense that it can be used offensively and as an escape. Another tactic is once Rath is lvl 5 you use Caldarius to scare people, two or more if able, close together and rath can use knockup, silence, and then Dreadwind to hopefully finish 1-5 players off quickly. Be careful as Rath is a glass cannon and Caldarius is squishy.
Orendi+Boldur: This one has been fun and not something you would think of normally. With Boldur you can do what Boldur does best. Soak up damage and stun (when enemies get knocked into things). Orendi can hide behind large characters and although boldur is short you can still hide behind him (but don’t get in boldurs way) and pop in and out to deal damage and lay a shadow fire pillar. The thing about this combo is people are going to focus on Boldur because they’re not aware that he needs to be ignored majority of the time and flanked when able. He gets healing when attacked with his shield up later down his helix and he can wreck face against opponents that attack him head on. Use Orendi to attack from the sides while Boldur pulls “aggro” and stay mobile, like I said with all ranged characters you want to stay moving and bouncing when able.
I’ve had success and fun with Benedict+Oscar Mike, Alani+Galilea, Ambra+Galilea, Kleese+Multiple BB with shields
Don’t pay attention to anyone that says pocket healing, healing, Miko, or any other characters or strategies are cheap and OP. Those players just don’t know proper counters to these tactics so they will complain. Healing is a huge strategy in this type of game. Play who and how you wanna play so long as you’re playing as a team and playing the objectives.
At the end of the day, games are supposed to be fun so enjoy! 