Depends how you want it: 1,2, 3,4, 5,6, etc. or 1,4, 2,5, 3,6, etc.

First way, have your loop write two TDs, and on the second make sure you haven't gone past the end of your array.

For the second, you'll be writing the i'th element in column one and the ($#array/2 + $i)'th element for the second column.

Dealing with boundary conditions, rounding of integers, etc. is left as an exercise.