Church of Sigmar
Kingdom. Opponents cannot target your units with card effects unless they pay an additional 1 resource per effect.

Question: Does this stack of a player has two Church of Sigmar in play?

Yeah. Two constants effect do not interfere if not stated so.

Yes, they do.
Why? Because you don't consider them as a combo or as some dependable conditions or anything else.

It works kinda War Paint in 3 copies attached to one damaged unit or playing Runsmith's action one after another in one turn.

All effects stack unless otherwise mentioned on the card.

For example: Church of Sigmar + King Kazador results in needing to pay an additional 4 resources to target King Kazador (1 for Church of Sigmar, 3 for King Kazador's ability).

An ability that stacks as well is Toughness. If a unit inherently has Toughness 1, then it gains Toughness 1 from another source, it is now considered to have Toughness 2.

Another example: Having three copies of Grimgor's Camp in play results in a net reduction of 3 resources to your first Orc unit played each turn.

Anbother example: Playing two copies of We'z Bigga! will reduce the cost of the next orc unit played by 2 and give that same orc unit 2 damage tokens.