Regarding "Two currents advance in parallel through alternating positions."
The hint itself is saying two separate patterns are running simultaneously. For example, using numbers, pattern 1 might start as
1, 3, 5, 7
While pattern two, using letters for distinction, might be:
A,C,E,G
When combined, the pattern would result in
1, A, 3, C, 5, E, 7, G
To guess the next solution, find the next answer in the first pattern, then switch to the other pattern and solve for its next solution. For this example, that would mean the following is the correct solution
9, I
Hope this helps ^-^