Multiplexor/Spelling

Both multi (many) and plex (to entwine, braid) are Latin terms, and this dictates we should use the (Latin) suffix -or as opposed to the English suffix -er.

Along the same reasoning we have "corrector" for "that which corrects", which derives from a Latin term, where as "baker" is not ripped straight from Latin.

Caveat:I have no authority to speak on this subject though, and it appears both versions, "multiplexor" and "multiplexer" are accepted perhaps the latter more so than the former.