L1/L3 distribution mission dropoff destinations are guaranteed to be in the same constellation as the agent (except when the agent constellation is unusually tiny, in which case you may go to a neighboring constellation - there are very few instances of this happening). L2/L4 distribution mission dropoffs will be in the same or in an adjacent constellation.
As such, if you ever mission for two or more agents simultaneously, it is extremely important that you ensure the agents are in the same constellation, that way the dropoffs share the same exact dropoff pool (at worst, the L1/L3 dropoff pools will be a proper subset of the L2/L4 dropoff pools). Interestingly, this means that L3 missions may complete more quickly than L2 missions in addition to offering better rewards.
An T1 agility indy is recommended for doing distribution missions. Fit properly, they can hold enough to carry 2x L4 distribution missions worth of freight at a time. (Blockade Runners cannot hold that much, Deep Space Transports are way too slow to be practical, and both extremely expensive from a liability perspective.) You can see what the USIA mission runners use for distribution missions here.