It depends on a lot of factors of the position. But, ensure that 2 knights plays better than a knight and a bishop in closed positions. But, if you could break this position and clean some board, the bishop could be more stronger than knight. So, dependes on the perspective of the position and if you and your opponent are be able to manage pieces to make them stronger or weak pieces.
two knights vs. bishop and knight

Just played a game with this and a lot of pawns left on the board. I was wondering if the two knights was better as - once the double knight player's pawns are placed on the opposite colour to the bishop - the bishop seems to lose a bit of value...

In the opening it matters little.
In the middlegame, it depends on how many pawns are blocking the bishops.
In the endgame, if there are pawns on both sides of the board, knight+bishop is better. If there are pawns on one side, knight+knight is possibly better.
Bishop pairs frequently get their well-deserved praise. But knight pairs ca also be extremely deadly in a localized area. They protect each other, create double attacks.
But if you had to choose a general rule, prefer bishop+knight.
Are there any stats/research of the Larry Kaufman-type on this? Is there any advantage to having one bishop plus knight vs two knights? Does it depend on other factors, e.g. closed center, etc.?