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Either way works really. Pros/Cons to both really.
AFAIK a lot of groups target the ripper claw/fireball dragon first anyway, and you just have to be behind him when he turns to dodge everything he does if you don't want to pay attention to discern claw (one hand) from fireball. (two hand)
After that it's just on the tanks stacking them together to not blow each other up with the cleaves. Being together makes it so you can cover both with Asylum/Soil w/e. Each tank can put their dots on both mobs and all cleave attacks/AOEs hit both targets speeding it up.
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There's no mechanic associate to being close or far. So they can be close as long as they are only facing their tanks. Having separate them is safer, that's all.
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Almost always better to get them close together. Tanks, watch for your mob to do its cleave, then you have a brief window of time in which it's safe to reposition.
I've had groups that could not get through the check normally but were able to once they were able to do their AoEs and AoE LB instead.
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The two tanked mobs do have cleaves in regular Nidhogg. They don't hit as hard as in Ex, and don't give stacks of vulnerability, but they can still wreck any DPS and healers that get hit by too many.
If your group is skillful enough, grouping the dragons will result in a faster kill - but if your group is that skilled, chances are good that they don't NEED the faster kill; they'll finish off all three in plenty of time anyway. Groups that struggle to deal enough DPS are probably the sort that fail to dodge the Shadow Dragon's cleaves, tend to stand in aoes, and so on and so forth. These less-than-skillful individuals are highly likely to blunder into the Brob and Falak cleaves if the monsters are all packed tightly together.
With beginner groups, I recommend keeping them apart, just so the newbs have more room to maneuver and have an easier time recognizing the Dragon's cleaves and having plenty of space to get out of the way. For veteran groups, you can cluster the three mobs for funsies, but you really don't need to.
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They should be put together if your tanks are halfway competent.