cat source.txt
1;xxxxx;xxxxxxxxxx|xxxx;xxx;aaaaaa;4#a1x;x;x;x;xxxx#a2x;x;xx;x;xxx#a3xx;xx;xxx;x;xxx#a4xx;xx;xx;x;x
1;xxxxx;xxxxxxxxxx|xxxx;xxx;bbbbbb;3#b1x;x;x;x;xxxx#b2x;x;xx;x;xxx#b3xx;xx;xxx;x;xxx
2;xxxxx;xxxxxxxxxx|xxxx;xxx;cccccc;2#c1x;x;x;x;xxxx#c2x;x;xx;x;xxx
1;xxxxx;xxxxxxxxxx|xxxx;xxx;dddddd;1#d1x;x;x;x;xxxx
2;xxxxx;xxxxxxxxxx|xxxx;xxx;eeeeee;3#e1x;x;x;x;xxxx#e2x;x;xx;x;xxx#e3xx;xx;xxx;x;xxx
 
rm file*.txt

nawk -F\; '{if((f="file"$1".txt")!=of){close(of);of=f};p=$5;oFS=FS;FS="#";$0=$0;for(i=2;i<=NF;i++)print p"#"$i>>f;FS=oFS}' source.txt

cat file1.txt
aaaaaa#a1x;x;x;x;xxxx
aaaaaa#a2x;x;xx;x;xxx
aaaaaa#a3xx;xx;xxx;x;xxx
aaaaaa#a4xx;xx;xx;x;x
bbbbbb#b1x;x;x;x;xxxx
bbbbbb#b2x;x;xx;x;xxx
bbbbbb#b3xx;xx;xxx;x;xxx
dddddd#d1x;x;x;x;xxxx

cat file2.txt
cccccc#c1x;x;x;x;xxxx
cccccc#c2x;x;xx;x;xxx
eeeeee#e1x;x;x;x;xxxx
eeeeee#e2x;x;xx;x;xxx
eeeeee#e3xx;xx;xxx;x;xxx

{
if( (f="file"$1".txt") != of )  {
  close(of) # close current file if we switched to another output file
  of = f
  }
p = $5
oFS = FS # save current FS
FS = "#" # set new FS
$0 = $0 # rebuild fields according to new FS
for (i = 2; i <= NF; i++) print p"#"$i >> f
FS = oFS # restore FS
}

sort source.txt | nawk -F\; ...