Update: synonyms were added. Thanks to everyone who voted
Considering that the tag multiple-monitors has 3096 questions tagged right now, we may want to make all of those synonyms of this tag.
Back to the topic.Ubuntu Wiki articule and Arch Wiki article on this topic use "Multihead" and "multi-monitor" terminology. Gentoo Wiki as well. So these two are definitely synonymous. Multihead is frequently used term in context of X11, but judging from the number of questions and simple guess that non-technical users are likely to use "dual-monitor" term, I'd suggest using dual-monitor as the target and dual-head as synonym.
On the tangential topic, we may want to create multihead and multimonitor ( which can become synonym of existing tag multiple-monitors) , because the dual-monitor is very much specific to using two monitors, and yet there are other setups and ways to call it with more than two that can be done. Whether or not the later should be a synonym of the former it's up for a discussion.