The pump is only rated for 1 GPU and 1 CPU, plus it is already kind of loud. If I went custom now, it would be only reusing the CPU block, a new cylindrical pump/res combo, a 420mm rad on top, and a 240mm on bottom.
parallel blocks only count as 1/2 of a GPU total in terms of overall flow rate/restriction. the only drawback to parallel is lower flow rate in each GPU block, but given how much overkill water cooling is for GPUs in general, this is not an issue. radiators are extremely low flow restriction, too. the h220x would work.
no doubt a custom setup would work better in terms of flow rate and noise.