Decades ago, trying to save weight in the Tetons, I used a 10mm and a 7mm. Doing free-hanging raps over knife-edge lips had me reconsider this combo. The 7mm was just too thin and scary. I now prefer a pair of 8.5s when trying to minimize weight and still have the ability for full length raps.
Reasons to keep the two ropes within 3mm of each other are it lessens the chances of the thinner rope pulling through the knot joining them (if Overhand Bend aka 'Euro-death' knot is used and thicker rope is on the stiff side) , easier to help them run through rap device more evenly, and distributes weight more evenly between the two ropes.