I'm not sure how this should work, but maybe LW can get clarity from Paizo. Champions get Devotion Spells, which provide a focus pool with one focus point. Deity's Domain grants the deity's domain spell as an additional devotion spell.
Right now, the additional devotion spell from Deity's Domain does not increase the focus pool. Some people on the Paizo forums seem to think that additional devotion spells always increase the focus pool (up to the 3 maximum, of course). I'm not convinced of that, but there's language scattered across the rulebook that might support it.
Page 302 has some text about focus pools from multiple sources, and increasing the pool if it would have granted a focus pool. But given that the Champion already has a focus pool and deity's domain doesn't say anything about increasing the pool, I can't be sure one way or the other.
Right now, the additional devotion spell from Deity's Domain does not increase the focus pool. Some people on the Paizo forums seem to think that additional devotion spells always increase the focus pool (up to the 3 maximum, of course). I'm not convinced of that, but there's language scattered across the rulebook that might support it.
Page 302 has some text about focus pools from multiple sources, and increasing the pool if it would have granted a focus pool. But given that the Champion already has a focus pool and deity's domain doesn't say anything about increasing the pool, I can't be sure one way or the other.