If you want to climb both Mt. Rahm and Mt. Custer on the same trip then this is absolutely the way to go if you have time to do them on the same day. The high ridge walk provides an excellent viewpoint that changes character depending on where you are on it. Most of the walking is fairly fast Class 2/3. There is one Class 4 section on the southwest end where Mt. Custer begins rising up.
This route description assumes you have reached the summit of either Mt. Rahm or Mt. Custer by some other route and now wish to continue to the other peak not yet done. The low-point of the traverse is 8,200 ft at about halfway but there is not much feeling of upward or downward climbing during it.
Going from Custer to Rahm
This is the direction that is harder to routefind due to difficulty locating the crux Class 4 gully that leads down around the corner of Custer's steep Northeast Ridge. Because of this difficulty, the preferred direction is from Rahm to Custer. See this picture to get a sense of where gully is.
From the summit, scramble along the crest northeastward then descend eastward or southeastward down gullies and ribs for about 250 vertical feet (Class 3) until a steep Class 4 gully appears. This gully is between an upper steep section of rock and a lower steep section of rock.
Descend the gully about 40 feet (Class 4) to where a small horn appears on the left. Do not continue down past the horn. Instead, go left behind the horn (Class 3) to the next gully over. Scramble down the loose gully (Class 3) to a ledge. Take the ledge to its end then scramble up to the Custer-Rahm crest.
Continue along the crest all the way to Mt. Rahm. Mostly Class 2/3. The top of the crest is easier than traversing the south side but is more exposed to the precipitous north side. There are a few minor gendarmes to scramble around. There should be no difficulties.
There are two summits atop Rahm pretty close in height. The true summit is the one a couple hundred yards to the east, so if it's socked-in, don't stop at the first (west) one.
Time to make traverse = 1-1.3 hours depending on routefinding skill; Distance = 1.2 miles; Gain = -400 ft, +250 ft.
You will no doubt not wish to descend the way you came. Simply descend via Rahm's south side. There is one short section of Class 4 in a gully to deal with.
Going from Rahm to Custer
This is the direction that is easier to routefind. Going this way, you will be going up the Class 4 section.
From the summit of Mt. Rahm, hike west over the subsummit to acess the beginning of the traverse. Simply continue along the ridge keeping as close to the cest as possible. The crest is highly exposed to the north side but the scrambling is easier because the top is often comprised of flat rocks. Sometime small gendarmes exist that will force you to the south slope. The south slope is grooved by numerous gullies. Some are more problematic than others. All in all, it should take no longer than an hour to get from Rahm to the base of the Custer summit eminence.
One last notch (8,240+ ft) is encountered before the ridge tips up to an unfeasible Class 5 arête. Don't fret, you won't be climbing that. Instead, look left (south) of the arête. Immediately to the left is a narrow Class 5 chimney. That's not it either. Look farther down and left. About 50 feet lower than the notch is a gully/chimney/break that provides a route around the corner to the south side of the peak.
Descend from the notch to a horizontal ledge that goes toward the gully and melds with it in a Class 3 rubble pile. You will see the gully you really want to the left of the ledge gully. But how to get into it? It's base is not accessible. No worries. Continue up the ledge gully to its end. At its end look left. Behind a small horn you can make a short sideways Class 3 traverse into the left gully. Once in the correct gully, climb it to its top (30-50 feet of Class 4).
Once around the corner, simply scramble up and right in multiple gullies and on ribs to the summit. Class 3.
Time to make traverse = 1 hour; Distance = 1.2 miles; Gain = -250 ft, + 400 ft.
You will no doubt not wish to descend the way you came. Simply descend via Custer's South Ridge.
Ice axe in early season.
Maybe a 30m rope, harness, runners, etc. with which to rappel the crux gully if going from Custer to Rahm. The gully may not be protectable. I can't remember. It was fairly loose.