League-of-legends – When does building armor pen. become more cost-effective than building attack damage

Yes, it's worth it to pick another jungler. It does not necessarily have to be a counter-pick though, which probably needs some explanation:

• The advantages you gave for having a jungler hold for your team as well.
• What you labelled as 'more effective' does not really require a jungler at all (coordinate your team to kill the enemy jungler [wards!] or steal a buff)
• Your downsides are too situation dependent:
  1. If behind enemy lines, you better know what you're doing. You have to be sure that your champ can live up to the enemy jungler and you have to make double sure that all other enemies are on their lanes. While it is a downside, it's one for which the risk can be dealt with.
  2. If their jungler is higher level than you, then either you have to improve your jungling, or you got killed/hindered before. Shouldn't happen for the first creep round, unless someone messed up their early game (see below)
  3. same as 1. Do not get killed in the enemy jungle. The enemy jungler shouldn't be able to kill you (or you shouldn't enter his lair!) and if you get ganked it's your fault for losing track of your enemies. If you suspect only a single enemy missing that might come for you, you better be on your way back to your jungle.

Anyways, double jungle teams are quite a common thing, because neither team wants to ignore the advantages gained by having a jungler. However, it also means that your team should know how to deal with that situation. Here's a few tips for general two-junglers-play:

• Prepare for more ganks on both sides. For both junglers it's usually safer to gank, than to visit the other side's forest.
• Make your jungle starting save. Not getting harassed while you get your first buff is extremely important. The team should realize this and protect you. Place your mates into the forest access points near the river to watch out for early ganks. Only let them leave, once the buff is save.
• Of course, you can turn this around too and try to gank the enemy jungler with your team. Not just a single counter-picked champ, but 2+ champs.

As you can see, two junglers are possible, do make sense, and generally lead to very fast and intensive gameplay. Beware on random groups though, as a two-jungler game demands a lot more from all players and failure is easy to achieve in that setting.

I think that if your champion has good AD ratios to their abilities, then AD runes are better because they add damage to your skills as well as your attacks. If your champion has AP ratios or no ratios on their abilities, i.e. Tristana, Master Yi, etc., then you should get Armor Penetration runes instead.