Our collaboration didn't come from a deliberate search for a working partner. It grew unplanned from friendship and a ghost letter game. In some ways, we've done everything wrong. But our periodic tandem efforts work. Why?
The by-words are politeness, flexibility, consideration, and common sense. Before expressing an opinion think - how would I feel if someone said that to me? It's easy to have foot in mouth dysfunction when you don't consider how you say something. This is especially important when writing your opinion, because facial expressions and vocal tones are not there to soften the criticism.
Writing your answer also means you have time to consider what to say. So if you are cruel, your partner can reasonably assume that's exactly what you meant to be. Never say "That stinks". Don't attack. Don't criticize without giving reasons why you disagree, and stating what you think would work better. Politeness counts.
Even if you live in the same town we suggest you work primarily through correspondence. You can not rewrite life. You can edit and generally clarify a letter. You also give yourself cooling off time if your partner says something upsetting. Remember that the other person's opinion -- especially when it varies radically from yours -- must be taken seriously. Be aware that an excellent idea can hide behind a foot in mouth delivery.
Both parties egos must be in strict control. We all need a well-developed ego to survive rejections, especially when starting out. But, don't exercise yours at your partner's expense. A "Me First" attitude has no place in any relationship. Likewise, don't take things too seriously. Any criticism is just the other person's opinion, not a personal attack.
Flexibility is also a prime collaborative necessity. If you must have it your way, you are not able to collaborate. You must be flexible not only with story ideas, style, and methodology, but also about deadlines. Deadlines can not always be met. The unexpected must be considered.
For example, in early 1990 our writing schedule went down the tubes. A dying mother-in-law needed to be cared for. Three months later came the funeral, and all that goes with being the eldest son's wife. The physical, financial, and emotional onslaught made writing anything impossible for half a year.
Often less dramatic reasons occur for not being able to write. Everyone periodically falls into writer's block crevice (unless you are a self-aware computer, or an Isaac Asimov clone). Sometimes it's merely being temporarily burned out on a long term project. Be supportive with your partner. Some personal distress may be blocking them. If asked, let them unload, and give any ideas how you would handle the situation. Remember, you are a writer. You should to be able to "walk in the other person's moccasins." If they don't want to discuss their problem, butt out.
Remember that your joint project is not the only one the other person has. At times you need to set your collaboration aside and work on something else, because your partner is in a vital stage, and needs to give a different project undivided attention for awhile.
This doesn't mean you can't continue work on your joint effort. If you feel creative with it, then by all means be creative. But, don't be upset if your partner doesn't respond quickly. In fact, you may want to not send what you are doing until they are ready to be receptive.
Finally, consider using one pseudonym rather than both your names. This forestalls the question, which name is to be listed first, and gives your collaborative work a separate identity. If you do go with your real names, alphabetical order, or the person with the greatest track record first, is often the criteria used.
One way to choose a pseudonym is for each to make a first and last name list. Compare lists. Do you have duplications? Next make a new list with the duplications, and any names on the other list you like. Try different combinations. Make it a game. It is a relatively unimportant side-issue. What's important is a good story, not your literary persona's name. Use good sense, and keep things in proper perspective.