Swing timer is disabled when dual wielding. Instead disabling it, it would be better to use the MH.
Something like this:

Quartz\modules\Swing.lua

function Swing:PLAYER_ENTER_COMBAT()
swingmode = 0
end

function Swing:COMBAT_LOG_EVENT_UNFILTERED()
if swingmode ~= 0 then return end
local timestamp, combatevent, hideCaster, srcGUID, srcName, srcFlags, srcRaidFlags, dstName, dstGUID, dstFlags, dstRaidFlags, spellID, isOffMiss, _, _, _, _, _, _, _, isOffHit = CombatLogGetCurrentEventInfo()
if (combatevent == "SWING_DAMAGE" or combatevent == "SWING_MISSED") and (bit_band(srcFlags, COMBATLOG_FILTER_ME) == COMBATLOG_FILTER_ME) then
if (combatevent == "SWING_DAMAGE" and not isOffHit) or (combatevent == "SWING_MISSED" and not isOffMiss)
then
self:MeleeSwing()
end
elseif (combatevent == "SWING_MISSED") and (bit_band(dstFlags, COMBATLOG_FILTER_ME) == COMBATLOG_FILTER_ME) and spellID == "PARRY" and duration then
duration = duration * 0.6
end
end