I always replace them in pairs - for the price your not saving anything but costing YOU another service call time and labor
The 2 Prong coil should read 1,000 - 1,300 ohms on Pin 1 to 2
The 3 Prong Coil should read 1,300-1,400 on Pin contact 1 to 2
And 500-600 ohms on Pin 1 to 3
I buy in bulk so these coils cost me under $3.00 for the pair.