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Explanation:
awk '
{
if(NF==1)$0=p FS $0 # if there is only one field on a line then prepend the current line with p and a space
else p=$1 FS $2 # else set p to field 1, a...

Try:
awk '{if(NF==1)$0=p FS $0; else p=$1 FS $2}1' file