Your syntetic key is not really a problem, you can leave your model as is without loosing anything in performance, however you can manage your syn key as follow (I will explain with an example):
Table1: fields a,b,c,d,e
Table2: fields a,b,r,s,t
as you see fields a and b create a syntetic key:
TMP1:
noconcatenate
Load *, autonumberhash128(a,b) as key_tmp resident Table1;
TMP2:
noconcatenate
Load *, autonumberhash128(a,b) as key_tmp resident Table2;
KEY_TMP:
noconcatenate
load a,b resident TMP1;
concatenate
load a,b resident TMP2;
KEY:
noconcatenate
load a as aaa,b as bbb, autonumberhash128(a,b) as key_tmp resident KEY_TMP;
drop table KEY_TMP;
drop fields a,b;
rename fields aaa to a, bbb to b;
Hope it is clear
