running 2 circuits off 14/3 wire
As long as the red and black wires are connected to opposite legs of the incoming service, the white (neutral) will carry the difference of the loads, not the sum.
Therefore, the maximum amount possible would be 15 Amps, if one circuit was fully loaded, and the other one off.
You should measure the voltage between the black and red lines with both breakers on, to ensure they are indeed connected to opposite legs. You should see a 240 Volt potential difference there.
The best way is to use a double-pole breaker.
Using a so-called tandem breaker is not a double-pole, but a "twin" set of single poles connected to the same leg of your panel. Be sure you can tell the difference here.
Life is uncertain -- eat dessert first!!
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