Fehzor gave you a subtle answer, in which the two weapons synergize with each other. Let me give you a less-clever answer, consisting simply of one good bomb and one good gun:

* Dark Briar Barrage and Dark Retribution are high-damage, easy-to-use, party-friendly bombs. Get both, and carry whichever one seems more relevant to the current situation.

* Alchemers (once you master the ricochets) and Blitz Needle are powerful guns. Carry one of those. Since you already have Plague Needle, just use that, unless you're carrying DBB, in which case you need a different damage type. Keep in mind that the gunner update will be released "soon", causing us to re-evaluate many of the guns. So wait a while, before getting any new guns.

Shivermist and Plague will wreck anything you come across, as long as it's not a slime or construct or freeze-themed (and even then plague charge wrecks the freeze themed enemy's faces anyway)

While it's a good idea to find weapons that synergise well with each other, it's equally effective to simply use two weapons that are both effective by themselves.

Edit: I typed this and left it in my browser and by the time I clicked submit there were two more responses \o/

Vortex+Sudaruska/Triglav is da best. The triglav will freeze enemies but the bomb explosion and after-shock usually sorts that out, leaving enemies moslty mobile but also shocked and not-attacking, helping with any dangers that freeze might have in hindering kiting. Combine with a valiance or your choice of antigua line to finish them off, or bomb/charge again.