The situation you describe is considered to be a double foul. Rule 9-7 Penalty 2b applies:
RULE 9 SECTION 7
PENALTY2. For a double foul during a:
a. Live-ball play, a replay is called;
b. Dead ball, the penalty is assessed against the serving team followed by the receiving team with both teams rotating.
Therefore, at the beginning of the set, a red card is shown against the serving team resulting in a penalty loss of rally. The receiving team receives a penalty point and rotates. Then, a red card is shown against the receiving team resulting in a penalty loss of rally. The serving team receives a penalty point and rotates to serve with the score tied at 1-1. The set continues from that point.