is it nuts to do 2-camera combo for a holiday

[sin77]

If it is photography trip or your whole family also enjoy photography, then you should buy more equipment. Otherwise like what the above person said, just 1 camera set up with a good zoom lens (definition up to you) and perhaps plus a fast prime.

If really want a combo setup, my ideal one as follows:

Two bodies, doesn't matter which models.

Lenses would be 9-18mm, 25 f1.4, 75 f1.8.

You want a walk about zoom that can capture ultra wide angle, 9-18 will do the job.

Encountered low light situation or need some creative shots using thin dof? 25 f1.4 matches well enough.

Found something faraway? 75 f1.8. Not close enough? use digital 2x teleconverter. (If it is a Panasonic model, you still can set at 1.5x teleconverter.)
Having said, the alternative to 75 f1.8 would be 35-100 f2.8 if you want quality. Otherwise u can consider 45-175 as well.

 

[scroogee]

it's a sightseeing holiday with just the gf and another couple. gf indulges my photography hobby and likes to point out things for me to shoot, though she also shoots some with her compact (my XZ-1 got appropriated, hehe). we would never go on a package tour and usually go with a just very loose itinerary. typically, last day or so is spent chillin in just one place, a beach or some sleepy town, and i usually just leave the camera behind.
my guy friend will be bringing a D3100, likely with an 18-200, and partly why i'm encouraged to be a bit more experimental by going with primes.

 

[vainqueur]

I can use just my 9-18 even for portrait shots. 3 years ago, I traveled to Egypt with my first mft epl1 with just a kit lens and it was enough for me.

 

[swifty]

You're certainly not nuts. I've had 2 or more cameras for a number of years now.
I'm kinda in a similar situation planning for an end of year trip. To buy or not to buy a GX7 (third body in my case if I don't sell any of my other camera bodies).

Consider the following:
Day time sightseeing - Body 1 + 12-35/2.8
Body 2 + 75/1.8
9-18 in the bag
Since you already have an ultra wide, bring it if you don't mind the weight. Otherwise do a short pano with the GX7 and 12-35 for those situations where the 12mm end isn't wide enough.
This setup should cover all the spontaneous shots where changing lens might mean missing the decisive moment.

Night time: Bring just one body with a fast prime of your preferred FL (pancake if possible). I like to travel lighter at night.
Add a 9-18 if you must do nightscapes. In which case you're likely to have a mini-tripod and something to carry that in so the 9-18 can go in the same bag.