Method 1: Select just column 1, and apply numbering to it. Select just column 2, and apply numbering to it. Both columns will be numbered from 1. Right-click the number of the first item in column 2, click Set Numbering Value, and set a value that's 1 more than the last value in column 1. If you add or remove rows, you'll have to reset the start number in column 2.
Method 2: For a table that's, for example, 2 columns wide and 5 rows long, start by inserting a table that's 1 column wide and 10 rows long (so half as wide as you want, twice as long). Select the table, and then, on the Layout tab, click Columns > More Columns. Set the Number of columns field to 2, and set the Spacing field to 0. You'll have what appears to be a 2 × 5 table with continuous vertical numbering from one column to the other.
There's possibly a code-based solution, but these are the only methods I know of to do it otherwise.
Update: I should note, though it's probably an obvious point, that method 2 only really works when all the items are roughly the same length – specifically, when they are the same number of lines of text. Otherwise, the rows in each column won't line up properly. Still, if the number of lines differ for each item, I suppose you could set a single exact height for all rows in the table (at least as large as required for the longest item) to ensure alignment.