I am installing 2 240volt 60 amp circuits for tankless water heaters. Do these neet to be run in a conduit? I am using #6 THHN which should be ableto handle 75 amps in this application.

I'm confused: Two 60A services for a 75 A load? Why not a 75 A service?

I do believe that the THHN would have to be in conduit.

I think my post was not specific enough. Each heater will draw about 54 amps at full load.

What does the manufacturer instructions state? What is the load in Kilowatts of each unit?

You may have one circuit up to 120 amps to feed water heaters.

But back to the questions.

If your looking at Table 310.16, yes #6 THHN is rated at 75 amps in 90 Deg C column, however you may only use this column if the terminals at both ends of the circuit are rated at 90 Deg C.

I know the breaker will only be rated at 75 Deg C, so you must use 75 Deg C Column so the #6 THHN must be protected at no more than 65 amps, or the next higher rating overcurrent device, which will be 70 amps.

The circuit size must be rated at 125% of the load, 54 x 1.25= 67.5, so your breakers must be no smaller than 70 amps. See Section 422.10 (A) of the National Electric Code.