The phrase "are mostly divided" is correct and usable in written English.
It can be used when discussing how a group or category is primarily separated into different parts or classifications.
Example: "The opinions on the new policy are mostly divided between those who support it and those who oppose it."
Alternatives: "are largely split" or "are primarily categorized".
The Jats of India are mostly divided into two large communities of about equal size: one Sikh, concentrated in Punjab, and the other Hindu.
The Jats of India are mostly divided into two large castes of about equal strength: one Sikh, concentrated in Punjab, the other Hindu.
Americans are mostly divided about whether to allow guns on campus, polling from HuffPost/YouGov has shown.
According to this week's Washington Post/ABC News poll, voters are mostly divided on how Obama is handling taxes (45% approve, 47% disapprove).
The relationships between members of the epistemic community and outside actors are mostly divided along sectoral lines (health and agriculture).
Some studies evaluating the hippocampal effect by the MS stimulation are mostly divided into low frequency (<10 Hz) or high frequency (>100 Hz) stimulation [ 32– 32].
However, for the last two weeks, my time is mostly divided into "bottling" and "tasting room" days.
So the field is mostly divided between LeGarie and Cooper, who are engaged in a sort of silent rivalry.
One year ago, the country was mostly divided between those in favor of staying in the E.U. and those who wanted to leave.
But the emergence of the SNC largely demystifies Syria's opposition map, which is mostly divided into the old guard and the revolutionary youth organised under co-ordination committees.
What all this probably means is that the temporary drop in support shouldn't distract from the fact that the public is mostly divided down the middle on the law — with marginally more disapproving than approving, but with outright repeal remaining a minority position — and that this has been locked in for years now.
