Can I mix urea and DAP together and broadcast in one go?

You can physically mix urea and DAP just before application and spread them together, but mix only at the time of use and apply the same day, not days in advance, because the blend tends to get damp and sticky and can clump. The bigger point is that mixing them for one broadcast does not change the basic problem: DAP phosphorus works best placed in the soil near the roots at sowing, while urea nitrogen is usually better split across the season. So a single combined broadcast on the surface is convenient but not the most efficient use of either nutrient. If you want efficiency, place the phosphorus source at sowing and apply nitrogen in splits. If you do blend for convenience, keep it dry, use it fresh, and do not store the mixture. For quantities, follow your Soil Health Card or soil test and your KVK recommendation for the crop, and do not over-apply.