I would just take the 24-70mm 2.8 (although would be tempted to pack the 50mm 1.4 if going to spend a lots of time in museums, dark churches, buildings, etc). I travel often but just go out with my camera and single lens, camera strap wrapped around my wrist. Pants always have front zippered pockets to hold my wallet and extra battery(s) and that way I can get lost in the moment and shooting photos versus worrying about being pickpocket, or setting my bag down, etc.

As far as shooting photos, I have a simple rule to myself "Enjoy what I am seeing and experiencing, then take photographs to remind me of the experience later." If I spend all my time taking photos, I may have nice photos but not any memories of being in the moment.

Re what which lens to take on travel:
When I travel overseas I take one body and a 24-105mm F4 lens, two batteries, two large SD card, one charger, some pre-moistened lens cleaners and I do quite well. I have even quit lugging the flash along. I shoot raw and instead of using a long lens I crop the pic or use my "foot zoom" to get what I want. By all means document the highlights, but make sure that you ENJOY the trip.

Re pickpockets in Paris:
The threat is there and it is VERY real. They work in pairs, or more, in ANY crowded place. The Louvre is a MAJOR target destination. In the Louvre they start on the escalators by holding you at the top while pretending to read a floor plan of the facility. While everyone is trapped they go through your backpacks, fanny packs, pockets, etc. It is then that you begin to notice all of the zippers on the backpacks are open and ladies are trying to figure out who was groping them. Some are highly skilled and some work undistracted while on crowded elevators. Beware of people very apologetically crowding into elevators at the last minute. It is merely a distraction, the pickpockets are already on the elevator with you. Lesson: AVOID Crowds of people bunched up together.

Maintain your situational awareness:
Wear your passport (better a copy, leaving the original in the hotel safe) in a passport holder, around your neck, and UNDER your clothes, along with the bulk of your money, credit cards, and other valuables. Keep only a small amount of money and perhaps one credit card in your FRONT pocket with a rubber band around the credit card and folding money. Lastly, place a handkerchief loosely placed on top of your stash. This makes it doubly hard for a pickpocket to get to your valuables. Remember, every time you spend money the pickpockets make note of where you keep your money. I know that this seems so negative, but except for the pickpockets, Paris is a WONDERFUL place to visit and I can't wait to return! Bravo Zulu, Paris.
https://smile.amazon.com/My-JAXO-Premium-Family-Passport-Blocking/dp/B01MS0NV8H/ref=sr_1_14?crid=1KG3MKQR56CCS&keywords=passport+holder+for+men&qid=1557827304&s=gateway&sprefix=passport%2Caps%2C136&sr=8-14

There are many wonderful museums in Paris. However, if you go to the Louvre, buy your tickets online to avoid the 2-3 hour lines for walk-ups. During the tourist season they often sell out and you have been waiting in line for hours for nothing.
https://www.louvre.fr/en

We loved London and had no concerns there for our safety. The pubs and taxis are great!
Smile,
Jimmy T Sends

I agree with Linda but will go further: get a camera that will fit in your pocket and don't "focus" on getting the best picture ever of Big Ben or the Eiffel Tower. I am headed to London in a few weeks and will leave the DSLR at home in favor of a camera with a 20MB 1-inch sensor that will fit in my pocket. Which will be zippered. I am traveling with my wife, a non-photographer, and we are just going to relax and have fun.

As Europe Guide Rick Steves says: when you go to Europe, never take anything with you that you can't afford to have stolen. Take an under-the-clothes pouch for your passport and credit card. If you are going to take your DSLR, consider getting a strap from PACSAFE that has an internal steel cable so that it cannot be quickly cut with a razor. And always pay attention. Anyone who is distracting you - bumping into you, asking you the time, etc., is very likely getting ready to rob you - or their accomplice right behind you will.

DD,
Many of us, especially me, have asked exactly the same question as you. Once in a lifetime adds additional pressure. Linda is right on the money. It is the easiest and best answer for you to create a balance between maximizing your enjoyment and ease and weight. I can give you reasons to take the wider lens, longer lens, and faster lens. And, I can give you reasons to take a different set up. Bill's makes the most sense. But all of these other reasons suffer from one thing or another. You do not wish to purchase another set up. Taking additional lenses will make your load; traveling from place to place and daily carry more cumbersome and impractical (for you!). And, of course, if you take one or another lens; you will come up against the same issue..at least once....'oh I wish I had taken the other lens'!
So, true to life...every decision is a compromise. This doesn't mean that you will be giving up anything. It means that you make rational decisions based on your priorities and just go with the flow.
Take your 24-70. And you and your wife make this one unbelievable experience...and take some pictures that will remind you of the places you've been. Enjoy your holiday..Bon Vacance!
Oh, while in Paris..if you love rice pudding...go to Chez L'Ami Jean. It is the best in the world!