We have a shul with several sifrei torah. Is it appropriate to use one for weekday readings and the other for Shabbat, so they are both used as often as possible, or is it better to use just one of them? If so, why? I thought that it would be better to use both as often as possible, so as to give a zechut to the people who donated them, and to use them so that the light and air hits them more often, which is good for the longevity of the sefer. Is that true?
The reason one might think of to use one sefer, rather than both, would be because one of the sefarim is of superior quality, and it is therefore a hiddur mitzvah to use it. However, the more correct, and, indeed, the common custom is not to favor one sefer consistently over others, provided that they are all halachically valid sifrei torah (in good condition), but rather to use them all. The ratinoale you mention of giving the donors the merit of their sefer torah being used is a sound reason for this, but the general honor of the sifrei torah is also sufficient reason..
Sources: see Maharsham vol. 6, no. 3, who writes that all sifrei torah should be used, because using only some would be an affront to the others; see also Chashukei Chemed, Megillah, p. 331, citing from Rav Elyashiv, who writes that there is no obligation to use the more mehudar of two sifrei torah.