Fastload can only load one table per job. Increasing the sessions will generally increase the throughput of the load - the sessions can connect to multiple nodes to push data in parallel. All of the sessions are used to load the same table. The benefit of more sessions depends on the size of your system. The Multiload tool can load to multiple tables.
However, in general, you should use the newer TPT utilities (TPT Load, TPT Update). TPT Update is capable of loading to multiple tables.