Until v0.9.11 of nginx-rtmp recorder could only create multiple unique files per stream (record_unique option) or re-write the file with each stream restart. Now a new mode is supported - append mode. The new directive is record_append. With this setting on the flv file is appended on each stream start.
record all; record_path /tmp/rec; record_append on;
Note that stream timestamps at the border are written close to each other without any gap so that there's no suspension when watching the recorded stream.