Hey, so did some potato sacks a few weeks ago.

one of them has really had a big growth spurt which is cool, but I do have a question

In this particular sack there are 2 seed potatos, both have produced a plant and well one plant the stem is like 15 -20 cm high now, which makes me think its time to add more dirt to bury it like you're meant too, however the other is only a like 5 centimeters high above the soil, so do I just add dirt anyhow up to the point of the tallest plant, or wait for the other one to catch up?

Cheers in advance