First one has close range and kills everything but undeads and fiends.
Second one is ranged and kills undeads and fiends.
Perfect match.
But I have my doubts about armor is the damage boost from merc demo or mad bomber big enough?
In case of chosing merc demo, both bombs already charge a bit fast so the lack of CTR wont be big right or should I chase UVs?
If you want merc demo, you are going to have fun trying to get CTR VH on your bombs (expensive as hell, but extremely good). and you still need 2X Elite Boom mods for max dmg.
If you dont want to waste money rolling UVs/buying, then get Mad Bomber + Volc Demo helm + 2X Elite Boom Mods (a lot cheaper, but less potential)
Summary:
Lots of money? Merc Demo
No money? Mad bomber + Volc demo