Carry good earplugs
I always carry earplugs for myself and my travel partner. I recently discovered Classic soft c 2003 Aearo Company (http://www.e-a-r.com), which are used at the plant where I work. They are the most tolerable earplugs I have found as I really dislike anything in my ears. They really block out the sound, e.g., I can now sleep on planes, during a room-mate's snoring, or in cheap Mexican hotels where the neighbours are partying through the night. And, my room-mates appreciate not having to listen to me snore!
Joanne Harris, Maple Ridge, Canada

Pack wine glasses
My travel-for-two tip is to pack a wine corkscrew, plastic wine glasses (the ones where the bases pop off for easy packing), nice napkins, and candles so you can pick up some local food and enjoy and impromptu picnic for two in the park. France is a particularly nice place to do this. Cheers!
Diana, Ingolstadt, Germany

Share reading material
When I travel with a friend/s we check before we leave to make sure we are both/all bringing books that the others/ haven't read. That way we can each only bring one or two and have at least double that to read.
Linda, Langley, Canada

Share clothes
When my friend, Sylvia, and I travel to Europe, we each pack just one bag, which means wearing a lot of the same clothes several times. Since we're about the same size - and like the same colors - we avoid clothing fatigue by trading. Sick of that black jacket? Hey, wear mine!
Pat, Chicago, USA

Pack snacks
Carry food snacks. When two people travel together, they have two different time clocks for eating. I take along a food bar with protein, to hold me because my traveling partner usually likes to eat later than I can manage that low sugar drop.
Linda, Canada

Double your security
Each person should carry a photocopy of all security info. for themselves and their travel partner. This list should include passport #'s and expiry dates, creditcard #'s and contact #'s in case of loss, medical and travel insurance #'s and contact #'s at home, family #'s to contact in emergencies etc.. I have done this several times and there is great security knowing that if my copies of these vital info pieces are lost- another is on the trip with me! Also (thankfully untested,) it is unlikely that both of you will be the victims of thieves, loss etc. at the same time. It goes without saying that this list must be guarded just as your passports are, and assumes that you trust your travel buddy with your info. An extra set of passport pictures with the other person is also a good idea.
Charlene, PEI, Canada

Decide on must sees
Have a travel meeting at a cafe. Before we even go on our trip, we look at some travel books and discuss whether there's anything in particluar we each absolutely want to do. Once those few things are on the itinerary, we can wing it because the must-do's are taken care of and everything else is just icing on the cake.
Juliann, Cincinnati, USA

Vote on must sees
My sister and I live in two different cities (Detroit and Toronto). Before we take a trip together, we each make a list of "Things I Want to See & Do in __" (fill in destination), and e-mail the lists to each other. Our #1 sights/activities will be those that we both have on our list. We then 'vote' on the remaining sights/activities, based on the time available during our trip. It works every time!
Alison, Toronto, Canada

Splurge on toiletries and share
I travel with my little sister. We usually share basic items like shampoo or sunscreen. The twist is that we buy nicer-than-usual products to make up for the slight annoyance of sharing. They make us feel extra pampered for the whole trip. Even better -- we buy toiletries overseas so we can discover exotic new products at the same time. We also have clear job definitions based on what we do best: I'm the planner, accountant and ticket holder; she's in charge of communicating/negotiating/haggling with people. When a situation arises, we know right away who's in charge and we don't waste time arguing.
Catherine, Montreal, Canada

Pack one outfit in her luggage
The best advice I've ever had is to make sure you always pack an outfit in your companion's bag. Then if one of the bags is lost there are back up clothes and less stress for both of you.
Karey, Portland, USA

Who sleeps on the right?
Start your holiday with a small decision that is very helpful. At the beginning of a trip we decide which of us will take the right or left side throughout the trip. This eliminates discussion over which is the best bed and which towel is whose.
Jane, Boston, USA