Can 2 standard water heater elements rated at 240V / 3500W be connected in series and powered at 480V?

Are you designing a new water heater?

A better way to ask this question would be to determine the amperage needed for each heating element and then see if such a heater exists.

If the heater is a standard water heater, then you must use factory replacement parts for that unit. Anything else would be considered to be product tampering (using components that are not designed for the equipment.

Is this a commercial installation? I ask this because 480 V is a derivative of 3 phase circuitry.

Back again.

Some simple math (Ohms Law) says that at 240 Volts, each 3500 watt element would require 15 amps (rounded up). The aggregate would be 30 amps.

Residential water heaters are 240 normally 240 volt / 20 amp devices.

In your situation, you would require roughly 60 amps X 1.25 (continuous duty) totaling out at about 75 amp. Unless this is an industrial rated boiler, you would probably fry the guts of the water heater.

Electrically, yes.

However, the UL Listing of the heating elements and the appliance will be voided, and not be allowed to operate in a building in the USA.