Have had NZBDrone up and running for a short while using NZBGeek and getting mostly good results. Occasionally though an NZB will show up there later than on OZnzb which is a second site I have access to, or not at all or for some unknown reason just isn’t recognized as being there … so I’m wondering can I add a second Indexer and have NZBDrone checking both? Or can I only use one or the other or do I risk otherwise messing up my smooth running installation in some other way?
Actually, NzbDrone is designed to support multiple concurrent indexers. So that’s really not an issue. The rss feed of all enabled indexers will be checked and if doing a manual backlog search all indexers will be queried.
For performance reasons you probably don’t want to add too much. Aiming for about 3 is good.
As for Download Clients, drone will use the first enabled one, but there is little reason to have more than one enabled. (At least not yet, there are torrent clients coming.)
I personally use 2, but 2 or 3 good indexers (without a low API limit, usually paid), is what I’d recommend.
Both my indexers are paid and have no limits, things were just going along so swimmingly last I didn’t want to screw it up.
Thanks for the info!
I’m using one really good indexer that pretty much has everything the second it’s posted. Recently I added Animezb to that because while regular series are fine, some of the more obscure anime series take a while. With the combination of the two I haven’t missed an episode yet. 
I have a few enabled just to cover the bases in case one doesn’t have an episode or does but it is password protected or corrupted i have a couple good paid ones and then some free ones