This puzzles me a bit, and I cannot find the answer anywhere. Does bi-wings create essentially double the uplift as single wings?
In general, no it does not.
If by bi-wings you mean a biplane then the airflow above the lower wing and the airflow below the upper wing interfere with each other and lift is lost. The loss is kept acceptable if the gap is at least 1.25 times the wing chord, but it is still maybe 20% depending on the design. The advantage of the biplane is that it is light and compact, which at slow speeds can more than compensate for the loss of lift.
If by bi-wings you include tandem wings, then the wake of the fore wing creates a disturbed airflow over the aft wing, reducing its efficiency. This can be minimised by mounting the fore wing low and the aft wing high, but that introduces the risk of superstall at high angles of incidence. The advantage of the tandem layout is that it allows a large range of positions for the centre of gravity, giving high operational flexibility.
Nothing is 100% efficient, so it's probably not TWICE... Given the same airfoil and chord, you're probably going to get close to the same that you would get if you had a mono-wing that was twice as long. The struts that go between the wings are going to add some drag. The shorter wingspan of the biplane though will likely give a faster roll rate, assuming that the ailerons are placed the same distance from the fuselage and are the same size in both the mono-wing and biplane.