Obviously, numbers are infinate. At what point would two marbles touch each other, assuming that all distances can always be halved? ( 0.1mm, 0.01, 0.001, 0.00000001 etc) Its a maths homework problem!
I know what you mean but I'm not sure what the answer is. If they are moving together at a constant or accelerating speed they would inevitably touch. They only way they would never meet is if they were also decelerating at a rate which was directly proportional to the remaining distance between them.
If the two items, marbles or whatever, moved closer to each other by half the distance between them each time, no matter haw far apart at the outset, it wouldn't be long before you were measuring the gap in microns. However, they would never meet but you would be unable to measure the gap! Try it on a calculator, then on a piece of paper when your calculator runs out of numbers.
Enjoy your Maths lesson, and be grateful for a teacher with a sense of humour.