Problem:

Columns A & B contain two lists of numbers.
We want to create a formula that first retrieves a value from List1, then one from List2, and so on down the columns.

Solution:

Use the INDIRECT, IF, MOD, and ROW functions as shown in the following formula:
=INDIRECT(IF(MOD(ROW(),2)=0,\"A\"&ROW(),\"B\"&ROW()))