In the US we vote by state. Thus it is possible to be eligible to vote in two or more states. If you meet each state's voting requirements perfectly legal. Thus a person could vote in a Pennsylvania primary in May. Move to Texas and register to vote in that state's election come June or July. Move again and vote in a third state for the November election. All of this would be legal.
Now voting in an election in two different when the election is held the same day would be harder, but if the state' define who can vote differently then possible. For example someone retains his Pennsylvania n residency while in the military, but finds he or she also meets the residency requirements for the state he or she is based in, finds him or herself eligible to vote in both states. Thus under the laws of both states such a voter can vote in both states.
Remember we do not vote on a national level even for national office. We vote by STATE. Senators and members of the House are elected by the voters of their state. Even the President is elected BY STATE, not by how many votes are cast on National level.
Thus the reason the candidate was NOT charged was probably because he broke no law in either state for his votes were legal under each state's law.