awk

Simplified version sample output

FIRST SITUATION

one.txt

AA;00000;
BB;11111;
GG;22222;

two.txt

KK;WW;55555;11111;
KK;FF;ZZ;11111;
KK;RR;YY;11111;

Generate this three.txt output:

AA;00000;
BB;11111;KK;WW;55555;KK;FF;ZZ;KK;RR;YY;
GG;22222;

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SECOND SITUATION

one.txt

v;k
mmm;d;e;f;

two.txt

xxx;d;
yyy;f;
zzz;f;

generate three.txt in this way:

v;k
mmm;d;e;f;xxx;yyy;zzz;