Keep it at 6V and wire the two motors in parallel (+ of the existing motor also goes to + of new motor; likewise with -).
Note that the peak current draw will now be 2x, which may be marginal with existing wiring and/or may stress the battery.
Sounds like attaching another 12volt will do some […]
I see a ton of 24v motors on the chinese sites. ha[…]
Yes, You can run them in series but it's no[…]
You can test the motors by applying 12v straight f[…]