That is a poor idea for a few reasons. The ones that occur soonest to me are:
- The issues with fast charging are only tangentially related to available power. The primary problem is charging the battery fast without damaging it at or near the completion of charging. That is often addressed with various kinds of "intelligence", (implemented in software and/or analog circuitry), which is unlikely to work right when two chargers are contending with each other.
- It is not clear that an unknown charger design will anticipate your hookup and that the device will survive it. I would guess it might, depending on where that "intelligence" is located, but I see no reason for certainty.
You're wecome in retrospect.