LED mixing: 2 off-line drivers vs 1 powersupply + 2 buck converters
The easy way is to use an HLG-40H-36B (750mA) and HLG-60H-36B (1.7A) at about $30 each.

Powering 2 buck drivers with a single supply is a very valid method.
I use a TI LM3414HV which is a simple inexpensive circuit.
But that's too much like work for a one shot deal.

You can use an eval board, from Infineon Technologies Part Number VALLEDILD6150TOBO1.  LINK: Infineon  VALLEDILD6150TOBO1

This is a very nice buck driver for $18 each.  Up to 60V and 1.5A.



UPDATE:

I was not sure when you said "They are all 35V" whether "They" meant sets or LEDs.   I assumed sets because the rest of the sentence was about sets.  So you have two sets of three 35V LEDs?  

If you could find them the 2 buck drivers will likely cost more than using an additional LED power supply driver.  

The only driver chip I could find above 105V is the Diodes Inc. AL1676 
The datasheet example is an AC input but it can also be used as a high voltage DC-DC CC driver.  They do not have an eval board for this part.

The current balancing the strings will be less efficient as it is the equivalent of a linear driver. You could use the TI LM3466 to balance the three LEDs in parallel to keep the voltage down. 

But I don't think you want to design your own drivers.

The cost of two buck drivers is more than an ELG.

Your original idea to use the two ELG-100H-C was your best option. That is if you are powering it with over 200VAC, otherwise it is a 70W driver.  The HLG-120H-C is a good alternative with 94% efficiency (vs ELG 90%) and an additional two years warranty.