One way to have two E-steppers to do the same work should be to enable
Configuration.h. I have no experience in this myself, but it is a good starting point.
A second way is to actually connect the two identical steppers in parallel. That trick is sometimes used for Z-steppers. I don't expect the current consumption to be an issue. This will require soldering, unless your mainboard has two Z-stepper outputs and you only use one of them. Then move connectors and reconfigure pins.h (or similar file) to use the correct steppers for
A third way is possible if your mainboard has removable stepper driver modules. Remove the
E1 stepper driver from the socket and cut off the
STEP pins. Solder thin but insulated wires between the solder-pads on this one to the
E0 stepper. These are all inputs, so now the second stepper will follow the first perfectly. Soldering shall be done while the stepper driver modules are removed, otherwise the heat might ruin the sockets. (There are more precautions)