MY personal favorite double bless nation/theme is ... wait for it.

Jotun Woodsmen with Death 9, Nature 9.

The trick is being able to get up that high without crippling your research. But I've found it to be very effective.

But when using that particular blessing combination the Woodsmen are particularly nasty, long Lasting and have a good survival rate. Especially if you include them in normal squads of Giant Spear Huskarl.