hi all: thanks for helpful tips; followed various suggestions/'formulas' on here but still having difficulties finding a mixed booking BCN-GRU on SQ in C; second sector GRU-EZE on JJ in Y
Use multi-city and enter:

From BCN :: sq+ / f bc=C|bc=D|bc=I|bc=R
To: GRU

From GRU :: jj+ / f bc=Y|bc=B|bc=M|bc=H|bc=Q|bc=X|bc=N|bc=V|bc=W
To: EZE

Result: 'no flights found' although when searching separately - flights are available (C on SQ and M on JJ) - any advice?