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I'm guessing you mean comparatists. Both deal with language, though in very different ways.

Linguists study language in a methodological, scientific way. They study the articulation of speech sounds (phonetics), grammar (which encompasses much more than any English teacher has ever taught you), and meaning within language; other sub-disciplines of linguistics look at such things as how languages are genetically related, or how language is acquired and stored in the brain. Linguists may be interested in a certain language or group of related languages, or they may examine various languages or conduct experiments to learn about the structure of language in general.

A comparatist is one concerned with the field of comparative literature, which is a branch of the humanities. They are interested in studying literature from various nationalities, time periods, languages, and cultures, for reasons too nebulous for me to bother examining. Suffice it to say, it's not a science, and I would consider it more closely related to philology than linguistics. (There's a sub-discipline of linguistics called comparative linguistics which is interested in comparing languages to determine their genetic relationships. This should not be confused with comparative literature.)

As for the number of languages a professional in either discipline must know, it really depends on what kind of work you want to do. But there is in fact no requirement for a linguist to speak more than one language, though it is often useful to have some knowledge of different languages. The majority of linguists, however, are indeed multilingual (my linguistics professor speaks 7 languages, to various degrees of proficiency). In short, linguistics is NOT about learning foreign languages.

A comparatist, on the other hand, due to the nature of his chosen subject, must be able to speak different languages very well in order to examine firsthand different literary traditions. However, as I take it, it's possible for a comparatist to study works strictly within one language, but within different historical and national contexts. So, I guess multilingualism is not a requirement of either field.