After my experiences this past week, I would recommend putting one in Mountain Home and one in Burley. If you are going to have only one in Twin Falls, you will have to be completely charged there and then in either Tremonton if heading North or Boise if going South, and out-of-range for a 60kWh battery.
Oh, I'm sure it's completely doable with just the one station - if you crawl along along at 25 miles under the speed limit (55 mph). And, you shouldn't be doing road trips with the 60kWh, in any case - that's strictly a commuter vehicle. But, I left Boise with 217 miles of range and got to TF with only 23 left, running right at speed limit of 80 mph (with the exception of 70 mph through a construction zone for ~8 miles), with no memorable accelerations to accommodate passing - just a standard ho-hum trip. Wouldn't have made it to Burley without slowing down. And, leaving TF the next morning with a full 255 miles of range, got to Tremonton with 60 miles of range - again, right at the speed limit the whole time, no atypical accelerations. Which means that in both cases, leaving with <200 miles rated range means you have to drive under the speed limit.
That may be fine while we are pioneers of the West out here - I've certainly had my share of will-I-make it trips, where I had to count on 55 mph to have a ghost of a chance. But, for a finished corridor, to have to choose between spending an extra half-hour charging, twice, or being a road hazard because you are so slow compared to the rest of traffic that you are practically a standing object, in order to complete the trip, leaves a slightly sour taste. Stopping at Mountain Home and Burley both would be another 20 minutes, but since you would be charging less at Boise/Tremonton the total would be about an hour gain in the trip time between SLC and Baker City. And for those who own Teslas in TF (Thanks Cleve for letting me stop by! Things didn't go as planned but not your fault, you were a very gracious host), remember, the very last SC location you want installed will be the one in your town - because that's your base and you always have your car charged when you are there; it's just for road trips you need the network, and in that case having Burley/Mountain Home SCs would be very advantageous.
On the way back through I did a full charge at Tremonton, drove @65 mph to TF, charged for two hours to add 50 miles of range, and got into Boise with 4 miles of range left. I let myself run the AC for one rated mile for every 5 I drove - not conditions I would subject my family to, but they weren't with me, and I chose to do that rather than slow down even further. Once there is another SC to close this gap I will be able to leave the AC on and charge up to full in almost half the time - but I will have to spend at least an hour at the SC there in order to be able to drive on to Boise/Tremonton as if I were in an ICE vehicle.
Charging time between SLC and Pendleton with one more SC in Twin Falls:
-Tremonton - 60 minutes (200 rated miles range needed)
-Twin Falls - 60 minutes (200 rated miles range needed)
-Boise - 40 minutes (coming up from practically no charge at all)
-Baker City - 20 minutes
Total = 3:00
Charging time between SLC and Pendleton with SCs in Mountain Home and Burley:
-Tremonton - 20 minutes
-Burley - 20 minutes
-Mountain Home - 20 minutes
-Boise - 20 minutes
-Baker City - 20 minutes
Total = 1:40
So, adding two SCs instead of one will drop the charging time for that stretch of I84 by almost half - even with one more stop.