We're not dividing anything by infinity. We're dividing infinity by two in this case.
Take a 100 pound object. Two people pulling equal weight on either side are carrying 50 pounds each.
Now take the same object that weighs 100 pounds and let two people carrying it with one clearly much weaker than the other. The weight is still divided evenly but in this case on of them has to account for more than the other. Divide it by, say, 1.6(the point six representing the weaker man). This shows more or less the amount the stronger man would have to move while also showing how much the weaker man would have to move(subtracting it from 100 yields the result).
Infinity doesn't work that way. Infinity divided amongst 2 people, 5, people, 1.000000000000000000001 people would still result in infinity. That is the inherent nature of the number. One can't simply lift 250 pounds of infinity because infinity cannot be subtracted from nor evenly divided. Especially not in any particular quantifiable way.
I mean it's a fine feat. Lifting infinity, even with "help", is a pretty cool thing to do. Also fits the theme of a no limits character. In terms of hard math it falls short because the inherent nature of the number involved. I don't see why this one feat deserves so much focus when Supes has a) really good strength feats with solid numbers and b) it is a physical impossibility that is grounded in theory(like destroying a hole in dimensions and holding up the sky).