if you mix equal parts of super phosphate with urea you will get an analysis 23 N and 10.5% P but you will not get diammonium phosphate. If you use triple super phosphate and mix with urea you will get 23 N and 21 P this would be fairly ideal as a starter fertilizer 100 units per used per hectare and be separated 7 cm to the side and 7cm below where seed is placed. If muriate of potash was blended with triple super P and urea equally you would get a 14 N 14 P and 20 K this would give all major elements as a starter. The banding at 7 cm to side and 7cm below the seed would be sufficient to allay issues with salt issues except if the soil is saline by its nature and history. In none of these events is diammonium phosphate produced but the near analysis of di ammonium can be created by the blending in my opinion. If you are experiencing salinity and salting humate material could be included in the blend . Under your conditions the use of humate triple super phosphate muriate of potash and humate would give an ability to allay any issue with salts which are ameliorated by humate. This might allow a higher starter rate and give a one application to optimize your cropping result. Trialing these would show the responses and any potential issues. Urea will acidify the soil but if your soil is on the alkaline side this may be favorable.

If you want to prepare a physical mixture of different fertilizers, you must have to know whether they are compatible for mixing or not. I think Urea should not be mixed with super phosphate fertilizer. Second thing, urea contains nitrogen in the form of amide not ammonium, however, it gives ammonium upon hydrolysis. you may prepare a fertilizer mixture having N and P content similar to DAP but DAP can't be prepared by physically mixing N and P fertilizer.

The abilty to use a blend can be determined by mixing and checking for compatibility. The different sizes and density of the materials can make the materials to separate when they are agitated.