I recently had a 5K/10K combination where they had to use a common start/finish for both events, and there was a limited, almost long enough space for an out-back for the 10K.
Stretching the out-back to the limit, even adding a big circle on that TA for more distance, I still had to leave the 5K about 15 meters long.
For the record I didn't hear any complaints about that. But then the actual distance was stated on the map and the cert.