I think ATI has more experience at it.
Neither have done much in-house work on multi-monitor implementations in any case... I'd wager ATi has done a bit more, but the overwhelming majority of their tech has been directly licensed from Appian
I'd lean towards nVidia as having a margnally better Multi-Monitor implementation on the driver level.... though ATi's still holds the crown from a hardware level implementation. Their VERY close though, I'd say nV has a slightly easier and very very slightly more capable implementation though.
That said, both are still quite a ways behind Matrox or Appian's full breadth of capabilities.
I'd say the difference is close enough that I'd definitely still be more inclined to get the R9700 over the Ti4600 as it's clearly the better chip in virtually every other respect.