Technically speaking you don't even need a return ticket when traveling to the Schengen Area. The relevant regulation (Schengen Border Code, Article 6) only lists the following related requirement:
(c) they justify the purpose and conditions of the intended stay, and they
have sufficient means of subsistence, both for the duration of the
intended stay and for the return to their country of origin or transit
to a third country into which they are certain to be admitted, or are
in a position to acquire such means lawfully;
As for traveling between the Schengen members states - no, it's not necessary to have an onward ticket of any kind. You do need to have a valid visa and you can't exceed the 90/180 limit, but you won't pass border controls and you won't be asked for any sort of supporting documentation. Such checks are only done at the external border.