AC: Powering a 25 amp appliance from 2x20 amp breakers
You so not want to do this. If you have a true 25A load then install the proper 30A or 40A single breaker and the proper wiring for that amperage (10AWG for 30A breaker or 8AWG for 40A breaker) in a single circuit to a listed receptacle rated for that current rating.

Here are some of the problems that you have with what you propose:


There is never any guarantee that both parallel breakers will split the current equally.
If one breaker goes open on a current overload the other breaker and circuit is left trying to source your 25A load through wiring that is rated for 20A or less. This can cause overheating. One would hope that in this case the second breaker would open but with just a 5A overload on a 20A breaker it may not trip right away.
In the case that you are on a split phase AC mains power system (such as used in the USA where 120VAC per phase is available) you could mistakenly try to parallel up two 20A breakers that were on opposite phases. Doing so would result in a direct short across the 240VAC and could cause lots of sparks to fly. 


So....again do not do the kludge that you are proposing and install the proper single breaker/wiring/receptacle to support your 25A load in a safe and reliable way.