Visited 7th September 2002: Neither of these stones is now standing, and there appears to be some uncertainty as to whether they are Bronze Age standing stones, marker stones, or just conveniently placed lumps of rock. Similarly, although they share a name, there may be no connection between the two stones.
That's the scepticism over with. I think that these two stones could be the real thing (i.e. Bronze Age standing stones) based on their proximity to the river and the meeting of two routes through the mountains. The stones are named after the nearby lake, which is a 19th century lead and silver mining reservoir.
The smaller of the two Pond Nant y Cagal Stones is to the south (SN72709034) and is accessible via a footpath that has been churned up by 4x4s. The path crosses the road, and you can park up almost at this point. Follow the footpath southeast (i.e. not in the direction of the lake) for a few hundred yards and you'll come to the stone on your left. It's not very big, so don't start out looking for a gigantic monolith!
The larger of the two stones (to the north) is adjacent to a cattle grid. You could park by the cattle grid if you wanted to, but we just walked up from the other stone. This second stone is a large lump of white quartz, which sits conspicuously on the hillside to the west of the road (to the south of the perimeter fence that runs down to the cattle grid). You can clamber up to the stone, but it's a bit steep, so not ideal if you're not feeling agile (like Lou, who is 8 months pregnant). The views from up there are beautiful though, with the lake and Pumlumon making a stunning backdrop.
I'd recommend visiting these stones if you're passing through, or if you're very enthusiastic about standing stones, but not if you're looking to visit a site that is still intact.