It is possible on older iOS versions. However, it will need to be done manually using coordinates as far as I'm aware. There is an existing helper repo I found here that might help (I've not used it though) https://github.com/superz515/UITestingMultitaskingHelper.
iOS 15 makes this slightly easier with the multi-task support UI.

If you observe the springboard application in XCUITest process you will see:
Default state:
Button, 0x600001a02d80, {{1004.5, 667.0}, {14.0, 32.0}}, identifier: 'top-affordance:XCUITestSplit', label: 'XCUITestSplit, Multitasking controls'
After tapping the control:
Button, 0x600001431340, {{968.0, 609.0}, {40.0, 40.0}}, identifier: 'top-affordance-full-screen-button', label: 'Full screen', Selected
Button, 0x600001431500, {{968.0, 663.0}, {40.0, 40.0}}, identifier: 'top-affordance-split-view-button', label: 'Split view'
Button, 0x6000014316c0, {{968.0, 717.0}, {40.0, 40.0}}, identifier: 'top-affordance-slide-over-button', label: 'Slide over'
After tapping an option:
BannerNotification, 0x600001a38000, {{966.0, 574.5}, {50.0, 217.5}}
Button, 0x600001a380e0, {{966.0, 574.5}, {50.0, 217.5}}, label: 'Split View, Choose another app'
Icons example
Icon, {{0.0, 0.0}, {0.0, 0.0}}, identifier: 'Files', label: 'Files'
Icon, {{0.0, 0.0}, {0.0, 0.0}}, identifier: 'Reminders', label: 'Reminders'
Icon, {{0.0, 0.0}, {0.0, 0.0}}, identifier: 'News', label: 'News'
Icon, {{0.0, 0.0}, {0.0, 0.0}}, identifier: 'Calendar', label: 'Calendar'
Icon, {{0.0, 0.0}, {0.0, 0.0}}, identifier: 'Reminders', label: 'Reminders'
Icon, {{0.0, 0.0}, {0.0, 0.0}}, identifier: 'Maps', label: 'Maps'
Icon, {{0.0, 0.0}, {0.0, 0.0}}, identifier: 'News', label: 'News'
Icon, {{0.0, 0.0}, {0.0, 0.0}}, identifier: 'Settings', label: 'Settings'
Icon, {{810.5, 155.0}, {95.5, 77.0}}, identifier: 'Shortcuts', label: 'Create XCUITestSplit split view with Shortcuts'
Icon, {{810.5, 353.0}, {95.5, 72.5}}, identifier: 'Contacts', label: 'Create XCUITestSplit split view with Contacts'
Icon, {{810.5, 548.0}, {95.5, 75.0}}, identifier: 'Magnifier', label: 'Create XCUITestSplit split view with Magnifier'
Example
The above can get you in split view pretty quickly.
However more control over how many columns requires moving the divider which is a bit more work (as I mentioned earlier), and you also need to consider resetting the state so the app doesn't launch again in split view.
func testExample() throws {
let springboard = XCUIApplication(bundleIdentifier: "com.apple.springboard")
let app = XCUIApplication()
app.launch()
springboard.buttons["top-affordance:XCUITestSplit"].tap()
springboard.buttons["top-affordance-split-view-button"].tap()
springboard.icons["Shortcuts"].firstMatch.tap()
}
Gif (tap to watch, forgive orientation)
