Actually, between catiajim and KooKoo, you have the perfect solution to this problem.
The only thing that may negate the use of the reflect line, is if the part is normal to the plane on all sides. It doesn't know where to put the reflect lines, in that case, and you won't get anything. Then, it's just as easy to manually create a plane, and intersct it with the part body. The resulting intersection is your parting line/neutral element, and can be updated by shifting the plane. (hint - do not use the 'datum element' option to make "dumb" geometry, and you could use a sketch also - keep the yellow projection elements, and then if you update the part, the parting line updates with it)
To draft both sides simultaneously, you just need to use your parting line as the "neutral element", and when you click on the "more" button, you will see 2 options, both of which you need to select:
1) parting = neutral
2) draft both sides
Do this, and you should be good to go.
Hope this helps.
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