Since both of them had the same number of highlighters at the end, we can tell that:

Difference between Han and Ho at the start = difference between highlighters given

So we know Mrs Han had 18 more highlighters than Mrs Ho.

Since we also know Mrs Han had 2 times the highlighters of Mrs Ho, we can say:

Mrs Han: 2 units
Mrs Ho: 1 unit
Hence, 1 unit = 18 (difference)

Mrs Ho had 18 highlighters, then gave away 12; leaving her with 6 left.

So they both had 6 highlighters left :)