Those would not be the standard rules, no.
Generally speaking, off-hand attacks don't take a separate % activity. Instead, you pay for the normal hand activity as normal, and then get the seconary attack as a bonus. But of course all these attacks come at a penalty if you spend less than 100% activity. The penalty is -1 per 1% less than 100 you spend on the attack.
So, in your example above, if you spent 50% activity you could attack at -50 with your main hand attack and also attack again with your left hand at -50 too. If you were not ambidextrous, that off hand attack would suffer an additional -20 penalty, for a total of -50 for the main attack and -70 for the off hand attack.
Parrying reduces the bonus for all attacks simultaneously. So if you parried with 50, that takes an additional 50 off everything. Movement does the same, so if you moved 50% of your movement, that's another -50 to all attacks in the round.
See the rules in Arms Law for a breakdown.