Automatic A-B 240 V switch
There's no need for fancy-pants stuff here, a subpanel will do the job just fine

Since you have fat wire (6/2, although 8/2 would work just as well) running to the existing condensor, what I would do is install a subpanel in place of the existing (presumably fused) A/C disconnect and put breakers for the A/C and well pump in that, instead of rigging up some goofy switching arrangement.

The cheapest option here would be a "spa panel" sized enclosure (while you can put a larger subpanel in here, the restriction to 240VAC only imposed by not having a neutral at this location severely crimps its utility) -- an Eaton BR48L125RP will do the trick here and for not much money, either.  You will not need any accessory ground bars -- leave the existing bar bonded to the enclosure (or bond it if it did not ship that way) as it will be your ground bar due to the lack of a neutral here.

Once you've turned off the breaker for the A/C, undone all the wires, removed the existing A/C disconnect box, and mounted the new subpanel with the existing cables fed into it via knockouts, you can then wire up the incoming cable with the white and black to the two incoming hot lugs on the subpanel, and the ground to the big lug on the terminal (ground) bar.

From there, you can then fit a BR220 for the A/C (replacing what I presume must be 20A fuses in the existing disconnect and serving as the air conditioner's disconnecting means) and a BR240 (to handle the starting current as per the pump manufacturer and NEC 430.52(C)) for the well pump circuit, which will need to be wired using 10/2 cable out to the pressure switch and from there to the control box.  The hot wires to the existing air conditioner condenser get landed on the lugs of one breaker, while the black and white (hot) wires of the 10/2 cable for the well pump get landed on the lugs of the other breaker, and the bare ground wires go to the terminal bar screws.  From there, you can wire up the well pump circuitry (pressure switch, control box, well pump) as per the manufacturer's directions.  Don't forget to torque the terminal screws on the breakers and subpanel to the manufacturer's specs using an inch-pound torque wrench or screwdriver, as well!