The task can be easy done by sed

sed '
    N  #append next line (operate with `line1\nline2`);
    /\nalt_id/s/\([^0-9]*\)\n.*/&\1/
       #if next line starts with `alt_id` the append end of present line
    P  #print present line (all before `\n`)
    D  #remove all before `\n`, starts from begin with remain part (line2)
    ' file

The task can be easy done by sed

sed '
    N  #append next line (operate with `line1\nline2`);
    /\nalt_id/s/\([^0-9]*\)\n.*/&\1/
       #if next line starts with `alt_id` the append end of present line
    P  #print present line (all before `\n`)
    D  #remove all before `\n`, starts from begin with remain part (line2)
    ' file

Other way is use hold-space

sed '
    /^alt_id:/G #if line starts by `alt_id:` append hold-space
    s/\n//      #remove `\n`ewline symbol
    t           #if removing success pass further commands (go to end)
    h           #if no (for other lines) copy it to hold-space
    s/\S*//     #remove all non-space symbols from start till first space
    x           #exchange hold-space and pattern-space ==
                #+put resedue into hold-space and return full line
    ' file

The task can be easy done by sed

sed 'N;/\nalt_id/s/\([^0-9]*\)\n.*/&\1/;P;D' file

The task can be easy done by sed

sed '
    N  #append next line (operate with `line1\nline2`);
    /\nalt_id/s/\([^0-9]*\)\n.*/&\1/
       #if next line starts with `alt_id` the append end of present line
    P  #print present line (all before `\n`)
    D  #remove all before `\n`, starts from begin with remain part (line2)
    ' file