You can use two reservoirs with pumps in series. Both reservoirs should be of the same model. Difference is that res on 2nd pump has to be only there to supply coolant - 1 outlet only without inlets. This approach has some short-term advantages when you want to make sure that second pump always have the coolant when you turn it on. One thing to remember tho: both res must be on same height. Otherwise reservoir which is higher will get all the air in the loop and res which is lower will slowly fill up to the brim.
My loop is configured that way that I made 2nd res removable after system is fully bleed and functional. There is nothing to gain by having 2 res feeding same loop. It doesn't matter if reservoir is 100ml in volume or 5L. Amount of spare coolant makes no difference on heat transfer or heat evaporated or flow rate.
From what I gather it's more beneficial to put DDC before D5 as DDC offers much better head pressure, but in the end such approach is misguided. D5 is cooled by coolant, DDC dumps all heat into the case. Get 2 D5s and forget about D5+DDC combo. I'm not saying it won't work. It will, but there is no point in mixing two pumps.