So what you have is a double not a two pole, cool. If you are using GFCI breakers then yes you need 12/2 to each because the neutral terminates on the breaker not the neutral bus and the neutral from each breaker goes to the bus. If you use the GFCI outlets instead of the breakers you will be able to use a 12/3 and it will be cheaper.
In order to determine if you go over the capacity is to take actual current readings of every circuit with everything in the house on not by adding up the breaker values in the panel.
120 Mixed Reef