We are beginning siman 3. The Chayei Adam writes that Chazal were concerned about a person performing multiple mitzvos through one process. This concern is known as ain osim mitzvos chavilos, that one should not perform mitzvos in bundles. One should not perform two mitzvos simultaneously, because we are concerned one will not have proper focus and alacrity for each mitzvah when performing them at the same time.
One of the primary places this concept is discussed is in Gemara Sotah. Rashi there explains that the concern is that a person appears to be doubling up their mitzvos in order to get them over with. This attitude demonstrates bizayon towards the mitzvos in the sense that the person does not appreciate the value of each individual mitzvah. Similarly, the Rashbam, in maseches Pesachim, writes that the concern is that it looks like the person wants to get the mitzvos over with.
However, the Chayei Adam does not give this explanation. His explanation is based on Tosfos in Moed Katan, who say that Chazal wanted a person to be fully focused on the mitzvah they are performing. Tosfos write that this issue is similar to the concept of ain me’arvin simcha b’simcha, that one cannot get married on Chol Hamoed because the seudos of Chol Hamoed and the chasuna would occur simultaneously and neither simcha will be given the proper attention it deserves.
Rav Moshe writes that Tosfos does not disagree with Rashi, but rather adds to Rashi. There are scenarios where one is doubling up mitzvos because they are trying to finish with them, and there are scenarios where the concern is not having proper focus for each individual mitzvah. Rav Moshe points out that the Gemara says we do not put multiple chasimos in one bracha (i.e., we do not end a bracha by describing more than one idea), due to a concept similar to ain osin mitzvos chavilos chavilos. In this case, the concern is clearly not the burden (even though it may not be proper to do so), but rather the inability to fully focus on one concept.
The Chayei Adam writes that the concept of ain osin mitzvos chavilos chavilos is derabanan, based on Tosfos in Sotah. If the concern of ain osin mitzvos chavilos chavilos was due to the bizayon, it would be a din deoraysa, as we have learned (shiur 1380). Thus, the fact the Chayei Adam holds that ain osin mitzvos chavilos chavilos is derabanan is congruent with his opinion above that the concern is one’s inability to focus properly on multiple mitzvos, rather than bizayon.
Ain osin mitzvos chavilos chavilos teaches that one cannot perform multiple mitzvos through one action, because we are concerned they will not give each mitzvah the attention it deserves. According to the Chayei Adam, this concept is derabanan.