These heaters may have a kit available allowing a single large circuit to feed the subdivided heat coils. I usually find it easier to run multiple smaller circuits especially if the heater already has separate breakers as the expected input. The thing that sucks is too many have an MCA of 56 or 57 amps. That means you need to use some cable other than NM if you wish to use #6. I just used 1" flex conduit and four #6 THHNs with a #10 equipment ground when I did mine (gets you up to 60A per conductor when factoring in bundling 4 current carrying conductors in a raceway).
Find the MCA rating on the furnace breakers to see what size circuits you truly need to run (when mine was installed, the package had about 20 different MCA labels and the HVAC installer was supposed to put the 2 correct ones in the right place -- I see a huge potential for error with that approach). That's a lot of heat for one house, and gas would have to be cheaper to run since it appears they already have gas service unless the pipe is too small or you can't vent it or something.
If you use the Optional calculation for a dwelling service, you can apply a demand factor to the heaters. You need to know how many stages are in each heater. There are most likely at least two since each has 2 breakers, but mine had 3 stages with two circuit feeds. You have two separate heaters so you could have 4 to 6 overall independent heat stages.
If 4 or less separate heating stages, the load calc is 60% of the heat strips. 220.82(C)(4)
If more than 4 separate heating stages, the load calc is 40% of the heat strips. 220.82(C)(5)
** This demand factor only applies to the service, not the branch circuits **
Even with this demand factor, I'd check your total service load calc, as 35KW of heat is a lot of electrical load.
