Follow up on my furnace... it has 2 240 60 amp brakers built in... do I have to use 2 240 60 amp brakers in my 200 amp service panel? 6/3 wg wire? That's 2 lines of 6/3 wg from my panel to my furnace?. thank you

Your system should have a rating plate telling you what you need. With 20 kw the heat strips will be around 84 amps the blower may pull around 5 or 6 amps.

Yes # 6 copper is the proper size for a 60 amp breaker.
You mention 2 pcs 240 Volts 60 amps breakers.
That means two circuits each of 60 Amps at the furnace.
Which requires 2 sets of wires rated for 60 Amps (=6 copper) from the furnace breakers to the service panel. And on the panel 2 breakers of first available higher rating than 60 Amps.
Don't forget the earth wires.
Success.
:)

Pl. Tell me 0totel summary of close & open loop in HVAC.