I definitely would recommend treating the entire tank, since that is a disease you definitely don't want to take chances with. I would not kill the two infected tetras. You can separate them to the other tank that is also being treated if you wish (I would if I was treating), just to lessen chances of spreading the disease. Gradually lowering temps (to 75*F or so) can slow down the progression of the disease.
Great deal of information about Columnaris here
Fish Columnaris | Fungus & Saprolegnia | Treatment & Prevention
Info on the active med ingredients (Kanamycin - KanaPlex, Nitrofurazone - Furan-2)
Aquarium Medications Part 2 | Antibiotic & Antimicrobial Treatments
Aquarium Medications Part 2 | Antibiotic & Antimicrobial Treatments
Note that using Furan-2 with KanaPlex is meant to be dosed at the same time/together in a combination to provide a synergistic effect to raise minimum inhibitory concentration (MIC) values, making the med combination more effective than when used alone. I am not sure how effective the combo is if you are already are using Kanaplex and then introduce Furan-2 a couple days or so after starting treatment. It should at least be better than only using one. Do not stop dosing Kanaplex just to wait for Furan 2. Once an antibiotic is started it should be dosed the entire/full course to prevent bacteria from building an immunity to the med.
Do note that Furan-2 can harm some plants (hardy ones like Java fern and Anubias will be okay, unsure about others) and is not safe for inverts.
I do recommend removing the meds out of the water via activated carbon/charcoal (turn on skimmers and UV's if you have them as well after treatment is complete) soon after treatment is completed as the meds aren't the gentlest (prolonged exposure can cause kidney damage and increased stress).