Can two 30 ampere circuits be combined in an outlet?
No.  While it might seem safe since each circuit is load-protected for it's gauge, you can have all kinds of problems.  

First:  Someone might work on the panel in the future and move a breaker and reverse the phase on one of the two circuits.  This would cause a 240v short circuit. 

Second:  Any fault to ground will likely fault to only one of the two grounds which would be undersized for the fault load.

Third:  Any difference in resistance in the two circuit hots will cause a difference in current in the two lines and cause nuisance tripping of one breaker.

Fourth:  Any difference in resistance between the two neutrals will cause a difference in current that could overheat one of the two neutrals without tripping the breaker.

Fifth:  As Ecnerwal points out, The appliance should trip at 40A, but if the current is balanced on both wires, the breakers may not trip until a combined load of 60A is achieved, causing a possible unsafe condition or damage to the new appliance.