Can I substitute two 14-gauge wires together for one 10-gauge wire? Using it from the generator RB-30 remote box, to the transfer switch inside a 6', 10-3 whip in the basement.
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).
The purpose of electrical codes is safety
As a general rule, electrical codes require the installation of all electrical equipment in accordance with the manufacturer's instructions.
Wiring equipment in a way contrary to the manufacturer's requirements or electrical code has the potential to cause fires and/or produce harmful and fatal electrical shocks. It constitutes a serious life safety hazard.
Nothing stops anyone from creating an unsafe installation.
The RB30 is not listed for the proposed wiring method. http://www.altronix.com/products/installation_instructions/RB30.pdf
It is unlikely that the transfer switch is either.