Wiring washer, dryer, tankless water heater
I think what you mean to say is you need a quadruplex 40/30.

The tankless would be fed with a #8/2 #6/2 to suffice the 33 amp requirement. ( black/white/ground ) unless for some reason it requires a neutral then #8/3 #6/3 ( Black/red/white/ground ).

The existing dryer 30 amp breaker would be replaced with the quadruplex.  The dryer #10 would connect to the 30 amp side, and the #6 would connect to the 40 amp side.

Note: The panel should indicate how many, if any, quadruplex it can allocate.  Also, the breaker obviously needs to match the panels manufacturer.