And you're more than welcome to use that interpretation in your games. But for me, it's a one-handed weapon, except for terminators. On the brighter side, I'm happy for it merely to be unbalanced when used in two hands...
DW are special, but they're not as special as a Chapter Master.
I would like to point out while there may not be actual models for it. In the standard space marine codex using a standard force organization chart. You can have 40 guys all carrying two thunder hammers, and they are all in power armor.
2 captains, 8 command squad veterans, 30 vanguard veterans.
So while it might not be something a beginning space marine can do, it is certainly possible for it to be done by ones other than chapter masters or special characters. Sergeants of any stripe can use two powerfists also, so that number is even higher.
It is your game and you can deny it if you want. However it is perfectly valid in the table top (which you seem to use as your filter), they just haven't made a model for it.