take a look at the chart in this link:
http://www.tomshardware.com/reviews/best-graphics-card-gaming-performance,3042-7.html
for example, 2x hd 3870 is as capable as an hd 5750 or 6750. when 3870 came out, two of those boosted performance but later single 5750 could do the same, 6850 would perform even better than 2x 3870.
It appears from the benchmarks that having a two card set-up boosts noticable performance, even for low end cards and this was the same case years ago, up until the introduction of the AGP interface.
i don't quite understand what you're saying. agp was phased out, pcie is the standard now. with pcie you have more choices. you can choose either 2-4 card setup or a single card setup. today's pcie cards are much more powerful than old agp cards.
I am asking, is will there ever be a shift back to single cards with comparable or superior performance to two cards?
two cards in combo will almost always perform better (in terms of fps) than a slightly higher tier single card. e.g. 2x gtx 460 vs gtx 570.
but if you use a single gtx 570 you don't have to worry about things like micro stuttering, ram bottleneck at high resolution. you can cool the single gtx 570 better than 2x 460. both setups work for different people. people who have a gtx 460 already and doesn't want to spend extra for a 570 can buy another 460 and sli. some other people can get a single 570 and be satisfied. it comes down to personal preference, budget.
then there are dual gpu cards. they are single cards containing two gpus in cfx or sli e.g. powercolor 6870 x2 or the new evga 560 ti 2win. high end cards like gtx 590 or radeon hd 6990 are single pcb, dual gpu cards that have two gpus with their own vrams. one can use those and get sli or cfx performance from a
single card setup.