I use dual-surveyors just to make logistics easier.
Making two designs is more cost effective, but not having to manage the ratio of types to send to each system saves quite a bit of micro.
This is especially useful if one uses reduced research and survey rate like me. Since survey ships use a precious naval shipyad, and it's hard to know beforehand how many you will need...
If we talk about ships (not fighters), it's just better to make survey ships that are 1000 or so tons larger, but are capable of performing both tasks. That way you do not have to juggle them in complicated way, and allocating more to important systems dscovered is smoother.
Also, I question the fact that single task ships are more efficient. Sure they are on paper, from a design point of view.... but you never know what you're going to find. Your more efficient ships will be useless if you are in a situation where half of them can't be used. Maybe you made 5 geosurvey ships... and then you find 2-3 systems with a ton of planets and want to survey those asap. With 5% survey speed, that takes a LONG time.
All your grav survey ships will not help in that situation.
I'm just saying, since dual-task survey ships are only marginally larger, but are a lot more flexible, I think that in c aurora they are definitely a more sensible options in a lot of cases.
IMO the biggest argument against dual-purpose survey ships is automation.
If my geo and grav survey ships are separate I can use standard orders to fully automate them - this is especially true if your like me and take in the 25% morale penalty into account when designing them. The only time I take control of my survey fleet is when a system has precursors/NPRs and I have to designate it as alien controlled
It takes almost zero micro to automate dual surveyors.
Time to survey some systems?
Give one surveyor standing orders
1) survey nearest body (or nearest five if you like)
2) move to system requiring geo survey.
Give the rest of your surveyors standing orders to
1) survey nearest survey location (or nearest three if you like)
2) move to system requiring grav survey.
Give them all whatever conditional orders you prefer for refueling/overhauling/etc.
If your gravs get too far ahead of your geos, switch a grav to geo.
If a geo runs out of work to do, switch it to grav.
If your gravs run out of work, you aren't exploring new JPs fast enough.