Hi welcome to the forum.
IMO I would play a pure cleric if you want to play a healer type. But if you really want to go Cleric/Warpriest, I would go 11 Cleric and 9 Warpriest.
The reason for this split is:
You will get a good feel for the Warpriest class by taking most of the possible levels
The really good Warpriest skills, Haste and Mass Heal are at levels 7 and 9
You will get more cleric spells than going 10/10
Implacable Foe is worthless at level 20
You will not get level 9 Cleric spells. I believe level 9 cleric spells will make you a more powerful character overall, not to mention the domains you are missing out on, but maybe you are up for more of a challenge
This will be a nice character as a support player in a group with your friends, since the Warpriest abilities do more for your companions than yourself. Just at the cost of power to yourself.