Can I substitute two wires for one heavier-gauge wire?
If the installation needs to comply with US NEC, 310.4 forbids paralleling conductors smaller than 1/0 (with some limited exceptions that do not apply to the described installation.)


  310.4 Conductors in Parallel
  
  Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG
  and larger, comprising each phase, polarity, neutral, or grounded
  circuit conductor shall be permitted to be connected in parallel
  (electrically joined at both ends).