If one motor is on a 60 tooth gear and the other is on an 84 tooth gear then they will be conflicting with each other, because they are both trying to rotate the wheels at different speeds. Assuming you are using blue cartridges and the wheels are on the 84 tooth gears, the motor on the 60 tooth gear will be trying to rotate the wheels at around 430 rpm, and the motor on the 84 tooth gear will be trying to rotate the wheels at 600 rpm. You need both motors to be on the same gear size for them to try to rotate the wheels at the same speed and not conflict.
Also I’m guessing you’re using 4 inch wheels because those are the only wheels bigger then 84 tooth gears, which makes your drivetrain’s max speed almost 90 inches per second if you’re using blue cartridges or around 30 inches per second if you’re using a green cartridge (for reference my team’s drivetrain is 61 in/s). The speed with blue cartridges is way too fast, and the speed with green cartridges is too slow, so I would recommend potentially switching out the gears on your robot so you have a more balanced speed. This post has a good list of drivetrain gearings: Catalogue of Drive Gearings