It is designed to allow two (or more) same-party members share a particular congressional seat during a single political term, which lasts four years (2+2=4). This is permitted by means of well manipulated Constitutional loopholes orchestrated by the political party who usually holds the executive branch seat (Head of State) and/or has congressional majority (both in upper and lower chambers).
It has been used mostly by the PRD party which coined the term themselves, but it may apply to the other two main Dominican parties.
The Dos más Dos formula works as follows:
During primary elections the party does not come to a conclusion as to who is going to be selected to represent the party in the next elections ballot for a particular Senate (or lower representative) chair. This is the result of internal payoffs and/or electoral "sabotage". It can also happen when there's a clear distinction between one candidate with the wealthiest sponsorship and the other who ranks better in the electoral polls.
The "salomonic" solution to this mess is to propose that only one should be registered in the official electoral ballot, and the other should wait for the second mid-term.