OK, why not. I use a +3 Mil hammer as my primary weapon, it's great for that purpose, but the secondary mode is only used when I press X accidentally. Why would I give up damage and knockdown for 1 speed or whatever it is?
Would it be 1 or 2 slots in secondary mode? Usually 2h mode of 1h weapons adds damage and speed, but doing so would make it FAR superior to the Mace if it was 1-slot. I say add the usual 3 dmg, 1 speed for secondary mode, but make it 2-slot (otherwise the Mace would be obsolete). OR keep it 1-slot, but keep the same stats as 1h mode (just press X for the awesome 2h animations/reach).