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Perhaps with the 600 wt you are trying to shift quickly. Patience......... is needed to allow the gears to slow down enough to shift. I used to double clutch to upshift on my truck but I just had to settle down and wait for it.
Now that said when the transmission is cold I have to shift pretty quick in order not to clash the gears but in about a mile or two I'm back to shifting sloooow between gears.
Yes you will need to double clutch to go from 3rd to 2nd. Most try it at too high of a speed. I was and still am guilty of that on occasion.