But the purpose of 1H+S is damagedealing with protection, not damagedealing with alternating weapons enjoying a huge inherent speed bonus over other combinations, which is DW's niche.
Thus, spending 2 DSI on enchanting both weapons in a DW setup benefits 100% of the game mechanics of DW all the time and spending 2 DSI on enchanting 1H+S benefits 100% of the game mechanics of 1H+S all the time. If you only enchant the weapon in a 1H+S combo, that combo benefits considerably less as a percentage increase to its purpose/mechanics supporting it when compared to the increase when you enchant both for 2 DSI.
Not sure what you are talking about. Let me demonstrate it with X being the effectiveness factor
2 handed / ranged: X - nothing to talk about
1h weapon with nothing else: X - ditto
dual wieldiong: X, then the other X, then the first X again, etc... it's never 2X, i.e. nothing ever stacks, that includes attack speed if the weapon is the source of it. (gauntlets of swift action and such will affect both)
1h weapon and shield: 2X, both weapon and shield stats apply at the same time (15% attack speed and the -15% recovery penalty) <- this is the only case where you get more bang for the buck.