Combining 2 lines together but "interlaced"

Keep fields of odd numbered records in an array, and update the fields of even numbered records using it. This will interlace each pair of successive lines in input.

prog | awk 'NR%2{split($0,a);next} {for(i in a)$i=(a[i] OFS $i)} 1'

Here's a 3 step solution:

$ # get one argument per line
$ printf 'a b c\nx y z' | xargs -n1
a
b
c
x
y
z

$ # split numbers of lines by 2 and combine them side by side
$ printf 'a b c\nx y z' | xargs -n1 | pr -2ts' '
a x
b y
c z

$ # combine all input lines into single line
$ printf 'a b c\nx y z' | xargs -n1 | pr -2ts' ' | paste -sd' '
a x b y c z
$ printf 'a b c d e\nx y z 1 2' | xargs -n1 | pr -2ts' ' | paste -sd' '
a x b y c z d 1 e 2