okay, if money is not an object here are a couple of the best dual setups:
GTX 690 4gbX2(has the highest bf3 and bf4 benchmark so far)£1060
GTX 780 Ti 3gbX2 £1060
AMD R9 290X 4gb X2 £780
AMD R9 295X X2(this is a pretty new card and it may outperform the GTX 690 but there arent many benchmarks for it yet)£2600
AMD 7990 6gb X2(an older card like the 690. i think they tweaked this to get the R9 290) 3gb=£420 6gb=£1800(maybe)
if you are say, on a lower budget like £400-500:
GTX 760 4GB X2 £410
GTX 660 Ti 2GB X2 £400
GTX 770 2GB X2 £450
AMD R9 280X 3GB X2 £400
7950 3GB X2 £400
and an even lower budget:
GTX 750 Ti 2GB X2 £220
AMD R9 270X 2GB X2 £280
GTX 560Ti 1GB X2 £200
GTX 650 1GB X2 £140
all these can be found on amazon.co.uk
there are alot more but this is best performance for price