Can I run two VESC controllers in parallel to double the power?

Do not connect the motor phase outputs of two controllers in parallel to power a single motor!
Each motor must be powered by a single, appropriately rated motor controller. If more power is required for one motor, use a controller with suitable voltage and current ratings.

Two separate VESC controllers can share the same battery when each controller powers its own motor. The controllers can be linked via CAN bus so they operate together as part of the same system.

The same principle applies to dual motor controllers. A dual controller contains two independent motor-control stages in a single device. Each side of the controller powers a separate motor; the two motor outputs should not be combined to power one motor.

Using two controllers, or a dual controller, with two motors can approximately double the system’s total available motor power compared with a single-controller, single-motor setup. The battery, BMS, wiring, connectors and cooling must all be capable of safely handling the combined power and current requirements.