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I just learned that you can now put more than one unit in a transport, unlike before it didn't matter how many models only 1 unit and independent characters could fit in.

I was planning on running 1x10 Banshees with Exarch and 1x10 Dire Avengers each with Exarchs in Wave Serpents.

Now I am wondering if it would be better to split them and run 2x5 for double exarch goodness. Obviously might not be as points efficient considering you are paying for 2 exarch powers and you can't double up, but the idea of Double Exarch sounds pretty intriguing.

You could put multiple squads in a serpent in the last book too.
I think with avengers I'd stick with the one ten elf squad. Giving the exarch the obsec boost for ten points is probably the best option but if you want to go all in twenty five gets you -4 ap on sixes. With guide doom etc they will kill just about anything you point them at. Biel tan gives you double exploding sixes for two CP, bladestorm and the Biel tan strat, to hit plus guide as a warlord trait. Plus the min battle focus move means you can peek out reliably.
Two squads of banshees is probably best and the the mirror swords two damage exarch power is great but if you do go Biel tan you can put exploding sixes on hits with the strat.
They're great units so you should have fun with them.