Power a 20A device from 15A circuits?
[Can I power a 20A load when available circuits are 15A]


There is no way to provide a continous 20A from one 15A circuit.

This is due to


The principle of conservation of energy.
Safety rules aimed at preventing your house/building burning down.





  Even if I can pull from two separate 15A feeds?


In most jurisdictions, there is no legal way to combine circuits to provide more power to a single load. 

The correct way to deal with this is to run a new circuit back to the main panel using wire sized for the load.



See also:


How to combine in phase 120VAC 15A 60 Hz feeds on a pair of outdoor GFCI outlets?
Can I substitute two wires for one heavier-gauge wire?
Can two 30 ampere circuits be combined in an outlet?


All of which essentially say no.