Where the two players live is the first thing worth checking on this page. An Illawarra pair quoting an Illawarra job has no travel to bury in the number, while the same booking taken by a duo coming down the M1 does, and the two quotes can look oddly far apart for the same four hours.
The other local wrinkle is the shape of the region. Almost everything sits in a narrow strip between the escarpment and the water, so a duo can work a Thirroul house party and a Shellharbour function room in the same week without much driving. Head inland or south past Kiama and that changes. The pool here is not deep, so start early and have a second choice in mind.
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Two wages sit under every quote, so the hours matter most, and setting up and packing down are part of those hours. After that it is travel and gear. Ask for a figure against the exact time you want rather than a generic evening rate, and compare it with where the players are actually based.
For a house, a restaurant or a function room up to around eighty guests, two players sound big enough without eating the floor space you wanted to dance on. Put a full band in the same room and you have bought volume you cannot use. The Illawarra also has far more duos available than large bands, so the practical choice often follows what you can actually get for your date.
As far as Berry and Nowra is routine, and Kiama is barely a trip at all. Past Ulladulla you are asking two people to drive the Princes Highway home in the small hours, which shows up as a travel charge and sometimes a room for the night. Name the town in your first message rather than the region, because the coast road adds up quickly.
Any working pair should be able to drop the volume and keep playing rather than stopping dead, and that is one of the real advantages of two people over a full band. Ask them how they handle the transition and what runs through the speakers during their break. A playlist through their own system costs nothing and keeps the room from going flat.
A duo is two specific people and they do not always live in the same town. It affects the travel in the quote and, more importantly, whether the person in the video is the person at your event.
An onshore wind off the water flattens a vocal and takes a light speaker stand over, and it does it long before rain becomes the problem. Pick a sheltered corner for an outdoor set and tell the duo where it is.
A duo used to plugging into a pub rig will need their own PA for a house, a hall or a paddock. Confirm what they carry and whether lights are part of it, because that is where two quotes often separate.