It depends on your firmware. If it's Marlin, I found a page with people discussing the same issue (and solutions) here.
In summary though, there are ways to configure any given firmware to have an E0 and E1 (possibly referred to as E and F), be driven simultaneously like G1 E30 F15
, just like you might call G1 X30 Y30
or G1 A30 B30
. The only reason it's not immediately possible is because it's a niche use case.