To design the primary beam, all that you need to know about the secondary beams is the nature of the reactions that they impose upon the primary beams. Obviously, if you're getting into the design of the connections between members, more information about the secondary beams may be required.
I'd add that the two work as a system, in terms of contributing towards limiting the overall deflection and vibration performance of a bay of flooring. Overall deflection/vibration of the entire bay of floor is therefore often the limiting factor in sizing both the primary and secondary beams. You will usually start with the secondary, then the primary to follow the loads from the start of the load path, depending on overall outcome, you might need to revisit the design until the combined system meets all your limit states.
You wouldn't go and design the foundations of a building before you knew what loads were on them would you..
