Your making the equivalency of what is calculated for BCPM for AMCAS is a) only acceptable science and b) accepted universally by all medical schools. Since LORs are defined by each school, that would be a false equivalency. It would be impossible to discern the strictness and perception by each adcom on this issue
As for the OP's question, Einstein's requirement is "the second letter should be from a member of a science department different from the reference who is writing the first letter." First, use references from the two professors who know you best and will write you the strongest letters, even of the same department. This will be of more value than a strict interpretation of the rule. Second, Computer Science or as the Engineering suggestion above would work. Engineering of all types would be likely viewed favorably by an adcom as well as Computer Science. In others words, dont get so focused on the strict literal fulfillment of the listed requirement to the detriment of the quality
