Typically one pot is all you need, if you are not getting enough range you can try experimenting with a higher value pot to give added range.
Another method to get extra range is to create a voltage divider. Typically there will be one of the two pitch bend points which is the main point, if you connect this to the middle pin of the pot and connect the other point to one of the outside pins of the pot then connect the remaining outside pin on the pot to ground. This will give you substantial range with a relatively small potentiometer however it only works on some toys.
One other thing to mention is I have encountered toys (including a Vtech apple which is the only Vtech I've done) where the pitch control resistor only gave me pitch up control and I had to find other methods to pitch down
I have bent toys where I used two knobs (one up and one down) but this was usually when there where two separate sets of bend points for pitch up and pitch down. Doing so with a single pitch resistor would get complicated.
Hope that helps