Remove newline depending on the format of the next line

Question

I have a special file with this kind of format :

title1
_1 texthere
title2
_2 texthere

I would like all newlines starting with \"_\" to be

Answer 1

If your whole file is like your sample (pairs of lines), then the simplest answer is

paste - - < file

Otherwise

awk '
    NR > 1 &&  /^_/ {printf "%s", OFS} 
    NR > 1 && !/^_/ {print ""} 
    {printf "%s", $0} 
    END {print ""}
' file