Never a dumb question...good units are not cheap so choose wisely.

If you have room and coin....two screens are better than one imo.

 If you have a stand alone Gps and sounder, if one packs up you can still fish, either by saved points  on the gps or using a marker buoy when you find a lump on the sounder. Nothing worse than trying to look for bottom on half a screen while running back to a mark

One for GPS and one for sounder. For GPS get a touchscreen, you wont regret it. So much easier to use

 Yeah I would hate to drive a car with the revs , speed and ancillarys all on one big gauge.

 I have a 5m boat but have ended up thru default with a furuno 628,   hds7 for gps and a hds5 for back up gps and to run the digital gauge cluster 

 thru the data cable from the yammy 4 stroke via a nmea backbone. ( imagine the set up  I would want if i had a real boat!!)

 Mates have joked about the twin battery set up needing a upgrade to triples just to run everything and a dash extension , but they are just overstating it.

 ( yes ok its a habit , I recently saw another hds5 that would look great out the back near the baitboard, via some sorta network hub)

imo
furuno for the sounder

lowrance for the gps /chartplotter

and definitly two seperate units as per johns logic above

 You won't go back. Just so much easier and efficient than trying to run a split screen. I thought my Lowrance Elite 7 Chirp wasn't too hard to use, until I went to the twin displays networked  together. But I didn't realise just how much better they were until I hopped onto a mates boat running the Lowrance, same as my old one. I was ready to put a hammer through it after 15 minutes.   just so much button pushing just to change a simple setting. The semi-touch Lowrances are a big advance on them, I've had a bit to do with one .

I would NOT go with pure touch for anything, after using both. One of my Axioms is the Pro, with Raymarines' Hybrid Touch, where you can use touch or customise some buttons to do what you want. The other one is the basic Axiom, pure touch, just harder to use with wet fingers on a rough day.  

If you go old school Furuno, you lose a lot of useful function, for no gain in deep water fishfinding ability, IMO, compared to a modern MFD running a 1kw xducer.  Just being able to look back at the sounder , see a lump you drove over without seeing just disappearing off the screen , and put a mark on it by just touching the screen over it, is gold.  Or being able to scroll back a long way to see if you have missed anything else.  Don't forget I'm an ex-pro user, I've tried them all, that Furuno tech is really getting old. You could put me in front of one of those 585/588/whatever, and it would be y2k all over again, I certainly wouldn't need to consult a manual.   Can't comment on the TZT series, but they look good from a distance. 

Running two networked MFD's, you have the option of running either display as any of the available options. They will both be fully configurable with all the available options. 

 Deffintley two screens, I run a HDS9 gen 2 touch for GPS, love the touch screen and wouldn't have it any other way. Furuno 585 for sounder makes for a great combo imo. 

just saw these replies, so sorry for the late feedback.

really great info, thanks heaps!