Can I wire multiple brushless motors in series or parallel?
so I'm thinking of wiring each phase of the two motors in series (i.e.
phase 1 of motor A is in series with phase 1 of motor B, etc.)

To wire the phases in series you need access to both ends of each winding. Maxon EC-4 motors only have 3 external phase wires and no way to separate the windings internally, so you cannot wire them in series.
However you can wire identical brushless motors in parallel, so long as you match the rotor angles precisely and the shafts are tightly coupled. Use the encoder from one motor only and the pair will act like a single motor.
In parallel the total maximum current will be doubled, and your controller will have have to handle twice the current of a single motor. So choose high voltage motors whose combined current doesn't exceed the controller's rating.