I have a 40 amp circuit. Can I use 2x10AWG wire for each conductor instead of 1x8AWG?
The reason (aside from "code says you can't") is that if somehow one of the two conductors breaks or (more typically) disconnects at one end, instead of a dead circuit (annoying but safe), you have an overloaded wire that can set your house on fire under normal load without ever tripping a breaker.
Hell no. Not least it violates the "All related conductors must be in the same cable or conduit" rule, causing imbalance/eddy current problems under normal conditions, plus the safety factor manassehkatz discusses.
However, if you really want to, and are willing to run some 3" conduit...
You must increase the conductor size to 1/0 AWG as a bare minimum, and realistically nobody does this until they're above 4/0, so you won't find a 1/0-sized...
specialized distribution panel specifically designed for paralleling on the supply side, and that type of equipment generally starts in the $1000 price range and 300A+.
You must follow all the other rules for paralleling.
You must get the AHJ to sign off on this, even though he's going to reject it out of general crazy-factor.
However, you can still feed the $1000 distro panel from a 40A breaker.