A Second Loaf

Do (most; all) bread recipes linearly scale, so if I want to bake a second loaf I just double all ingredient amounts?  YES if using a scale.

Is it better to mix a double batch and divide the kneaded dough, or mix a single batch twice?  

      Depends on your mixer or mixing arm and the size of your bowl.  Also loaf sizes vary.  Are you talking about a standard loaf pan size? Loaf mass may affect the fermentation rate as well as how the dough cools or warms up.  If following directions for one loaf, divide into loaves before cooling or warming up the dough.  

Any need to increase the mixer time for twice the amount?  Don't know, I mix by hand but usually more dough takes a little longer to mix. 

How do I adjust oven temp and time for baking twice the amount?  Pre-heat and rotate the bake half way through.  You may need 5 minutes or so with two loaves instead of one but if the oven is small it may need more time if very crowded.  If the oven is a mini oven, you may have to stagger the timing between fermentation so that one can be baked before the other.  This can be done by chilling one loaf right away while warming and raising the other.

I have a little micro bakery, so I regularly bake from one loaf (usually just when I'm testing a new formula) up to 12 or 14 loaves of one kind of bread. I keep the formulas in spreadsheets and simply scale up or down as required using the same percentages for each ingredient. No problem.

For mixing, I don't really change anything either. I sometimes mix one or two loaves by hand but, as I have small arthritic hands, I usually mix the dough in a mixer. One or two loaves are done in the KA mixer and four or more are done in the big commercial 30 quart mixer. Again, no change in timing (other than watching the dough and taking it out when it's 'ready').

For baking, the only change is to rotate and change position of the loaves halfway through baking (I'll even do this for one loaf; most ovens have hotter or cooler spots so a loaf will benefit from this). I have heavy granite stones in my oven so the heat is fairly even but I bake on two shelves. If I am baking more loaves than will fit in the oven at one time, I will refrigerate the second batch shortly before the first batch goes into the oven (i.e. just before it's finished proofing).

As Mini said-"as long as you use a scale"

There are tons of posts on scale vs measuring cups that I don't want to restart here but this is the time that scaling ingredients is far more superior to measuring. It allows you to maintain the ratios of ingredients necessary to maintain dough/loaf characteristics. Handling/shaping/fermenting times/baking times are affected by the difference in bulk. If you google around-esp on you tube, you can find videos (usually in Europe) of bakers mixing dough by hand in a LARGE trough (about the size of a rowboat). The bakers usually have big arms but it shows it can be done.

Doubling approximate amounts (in cup measures) only doubles the differences and the dough may turn out totally different. It works if you have the experience to adjust the other ingredients on the fly to achieve the desired loaf.