The Twin Peaks massif forms an odd isthmus on the northern border of the San Gabriel Wilderness. Connected to more pedestrian Waterman Mountain by a broad saddle, the massif forms a vaguely east-west rampart, steep on all sides, with the only easy access being via the saddle.
Though the summit of the East Twin is lower than the higher summits of the San Gabriels, the terrain is more like the Sierra Nevada or the San Bernardino Mountains. Granite outcrops and boulders pepper the ridge, and spectacular, often loose buttresses jut south into the upper drainage of the San Gabriel River. There are few peaks obstructing views to the south, increasing the feeling of elevation on the summit.
The easy route, a Class 1+ hike up the north face, climbs steeply between boulders and through a mixed forest of California live oak, white fir, ponderosa, and Jeffrey pine. The East Twin is the higher summit, and more distinct than the West Twin. The top of the West Twin is a gentle hog's back, nicely forested, and makes for sheltered, relaxed camping.
There are opportunities for technical rock climbing on the buttresses along the south flank of the massif, though careful route selection is critical. Much of this flank is very fractured and loose, like most of the San Gabriels. However, hiding amongst the scree are some neat outcrops of Sierra-like granite.
The ridge dropping east from the East Twin leads to Triplet Rocks - reputedly the hardest place to reach in the San Gabriels. Getting there requires an epic bushwhack and dangerously loose class 3 scrambling, culminating in a mandatory Tyrolean traverse to reach the highest of the 3 summit blocks.
From the junction of I-210 and CA-2 highways in Pasadena, drive up CA-2 to Cloudburst Summit, a high pass just west of Waterman Ski Area. Park in a pull-out on the south side of the pass, where the Pacific Crest Trail crosses the highway. Hike up a use trail leading directly to the Waterman summit lodge. Continue on dirt roads to the southeast edge of the Waterman Mountian summit plateau, then follow the Waterman Mountain Trail down a couple switchbacks to the Devil's Canyon Trail.
Turn right (west) on the Devil's Canyon Trail and descend a few more switchbacks to the Twin Peaks trail (unmaintained). Follow this down to the Waterman-Twin saddle, then up the steep north face of Twin Peaks. Near the top, follow the left fork of the trail, leading to the higher summit of East Twin.
Permits are required for overnight stay, and are available at the Chilao Visitors Center in Chilao Flats on CA-2, or at the Grassy Hollow Visitors Center ~2 miles west of the Big Pines Highway.
No permits are required for day hikes.
Best season is April to November. Snow on the very steep north side can be very dangerous in the evening, after it re-freezes into boilerplate.
Best camping for East Twin is on a ledge a few hundred feet below the summit. Look for a large split boulder - the ledge is just south of this boulder, and has a wonderful natural 'fireplace' in a small cave at the back of the ledge.
Gentler, more relaxing camping can be had on the long summit of West Twin.