Mark,
I have found that double swings have had good snatch test carryover, mainly in terms of developing a more powerful hip drive.
Without getting into GS territory, I've found snatch test preparation to hinge (no pun intended) on 3 main factors: hip drive power/endurance, grip strength/endurance, and wind. My goal for the 100 rep snatch test is to be able to do it comfortably at a quick, but not rushed pace (to finish between 4:00 and 4:15), with minimal hand switches, without putting the bell down.
When the test gets over 5 minutes, I also find that shoulder mobility/endurance becomes important too. If your mobility is lacking and you need to use too much muscle power to hold the lockout, shoulder fatigue becomes a limiting factor in a longer test. For this reason, I can crush the 100 rep test, but 200 crushes me.
For hip drive, double swing intervals are great. For hip endurance, I like long, sustained sets with a single 24kg, switching grips on the fly. But to be fair, I have not worked up double swings into longer work intervals like at the end of the One program.
For grip, I've found the best carryover from longer snatch sets with one hand switch, using 24kg and 28kg. I have not felt as much carryover from longer sets with lighter weights. For me, 28kg is a sweet spot where I am getting an overload effect, but don't have to cut my sets overly short or compromise technique. At my level of grip strength, I can only do very short sets with 32kg, using full power and an aggressive drop, so I don't feel I get great carryover.
For wind, I like longer sustained snatch intervals with 24kg. One protocol I like (Pavel once posted this on the RKC instructor's forum) is time ladders with 1 min. rest intervals: 1min snatch/1 min rest; 2min snatch/1min rest; 3min snatch/1min rest; 4min snatch/1min rest (switching hands at will). You can work up to this by starting with 1min work sets and progressing such as: 111, 1111, 1111, 1211, 1221, 1222, 1231, etc.
For the 100 rep test, I don't find any separate additional "cardio" training to be necessary.
Personally, I don't feel I've gotten a lot of carryover from heavier single swings, but I know that others have effectively used them as a key part of their preparation, so YMMV.