I think calling it a mixing drum would be more accurate. Continuous flow, or batched operation? Gas or liquid? Do the two streams mix easily? Will you need mechanical mixing, agitation, or mixing baffles and turbulent mixing, or will the two streams easily self mix on contact?

Probably should be sized to hold several hours of flow. Use a length long enough to ensure mixing has occurred by the time the flow has reached the opposite end, if the flow must be continuous.

You could probably reduce the length, if you can make that a batch mixing operation, ie. load a tank with a few hours of flow. Close it and open again when mixed.

Or do the same with two tanks and alternately draw from each one to make the op more continuous.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

What is surging? The gas phase or the liquid phase of the 2 phase feed ? Presumably, the downstream consumer demand is relatively constant, so this surge drum will have to accommodate the incremental volume flows during the time when surge occurs.