Combing two DC voltages, one high volt low current the other low voltage high current
What you are proposing is not feasible based on power considerations.  To use your example, a 500 VDC source at 100 ma can provide a maximum of 50 watts.  A 1 VDC source at 50 amps can also provide a maximum of 50 watts. Let's say you could somehow borrow 1 amp from the 1 VDC source and give it to the 500 VDC source.  Now you would have a 500 VDC source providing 1.1 amps or 550 watts from 2 sources that are limited to a total of only 100 watts.