Gate (double) stanchions are widely available, cheapest from yachtshop.eu (several designs/prices), but SVB has them at a premium too.
Our 40 Ocean came with gates but for us they're hardly worth having and very costly when we renewed the guardrails due to lots of extra fittings needed. We rarely even open them and usually just climb over. When med mooring you won't use them at all and they're a big waste of money.
If you do have less agile crew, they can make sense though. In that case I would absolutely position them at the shrouds, because it gives people something sturdy to pull themselves up with and they're also near the beamiest part of the boat. Additionally, they're far enough forward that when berthed bows in on a short finger pontoon you still end up stepping onto the pontoon rather than into the water!
Ours are right where the centre-cockpit has the foot step out of the cockpit, which seems nice at first glance, but they interfere with the Bimini supports and are too far aft for above mentioned short finger pontoons that you sometimes end up at.
Lastly, make sure you use good quality pelican hooks and not cheap snapshackles like Bavaria did. The snapshackles don't have enough leverage and are difficult to close and also harder to open. The crew hated them and when renewing them this year I spent a bit more on some nice pelican hooks instead.