I would not do this.
If the builder goes belly up you will be unable to enforce the agreement and be left with two mortgages to pay.
Also, having a second mortgage without the consent of both mortgage providers puts you in breach of both mortgages.
Further, the second mortgage provider has to be told about the deal and is likely to change (ie reduce) your mortgage offer because it would belive that the deal was an incentive designed to keep the sale price artificially high.