Don't worry too much about trying super hard to MLB a 2nd copy, just 1 additional copy is fine (although more is welcomed, ofc). The Dephyne hammers are basically fillers in the grid anyways because they can't be MLB, you'll want to replace them with Nephtis/Samael hammer whenever you can, and each additional LB is about 300 atk (400 with hammer bonus), which is not nothing, but not a big enough deal to really worry about by the time you have phantom grids going (particularly because dark - and if you're maining dark, you'll probably already have the hime hammers anyways). The biggest thing they do is allow you to power the phantom hammer's 30 assault skill, which will make up for having 2 filler, non-mlb weapons. Hell, even if you got zero additional copies, using a dark SR hammer as filler works fine. The most important part of the hammer grid is the 3 UE hammers (if you are thinking of splitting anything and bricking, these are what you want to use it on).

The truth is, until raid weapons get MLB (maybe years from now), going hard in these events is generally just not worthwhile. And since it's dark, just doing the minimum and keep waiting is imho the best approach. By the time they actually become good again (and I hope they do), the meta may have already changed drastically.