Foreign key on split unique columns
Found the answer. 
I was Trying: 

USE [FruitHistory]
GO

ALTER TABLE [dbo].[Fruit]  WITH CHECK ADD  CONSTRAINT [FK_Fruit_OnTruck] FOREIGN KEY([TruckNumber],[Vendor])
REFERENCES [dbo].[Trucks] ([TruckNumber],[Vendor])
GO

ALTER TABLE [dbo].[Fruit] CHECK CONSTRAINT [FK_Fruit_OnTruck]
GO
GO


But what actually worked was 

USE [FruitHistory]
GO

ALTER TABLE [dbo].[Fruit]  WITH CHECK ADD  CONSTRAINT [FK_Fruit_OnTruck] FOREIGN KEY([Vendor],[TruckNumber])
REFERENCES [dbo].[Trucks] ([Vendor],[TruckNumber])
GO

ALTER TABLE [dbo].[Fruit] CHECK CONSTRAINT [FK_Fruit_OnTruck]
GO
GO


Not Completely sure why but reversing the columns allowed it to be set