We have pigeons nesting on our balcony. They have two youngsters who will be three weeks old tomorrow. Today I discovered another nest on another part of the balcony would this have been built by the same pair?

They won't have built the second nest while having young to raise.

However if the second nest could have been there previously it's not impossible they built it previously but decided a new one would be better. or it could have been a different pair of course. Pigeons commonly have several broods a year (or even breed year round in some places) so you may find they stick around for a bit.