Dave is correct in that it's likely the second image consists of several cuttings in a single pot. The closest you can come to that w/o starting additional cuttings in the same pot as your single-trunk tree is to prune hard when the tree is near the part of the growth cycle where its energy reserves are highest and its ability to make food (plants make their own food during the process of photosynthesis when they convert the light's energy into carbohydrates/sugar) is also at peak levels. For healthy trees, this predictably coincides with the summer solstice, which occurs on June 21 in the northern hemisphere, and Dec 21 in the southern hemisphere.

The result you're looking for when you cut back hard is new branching to occur from the crotches (axils) of any leaves on the trunk after the chop, or from dormant buds immediately above the leaf (bundle) scars that occur where leaves were formerly attached to the trunk or branches.

Here's an example of a maple that was cut back hard and what occurred after. I probably scraped off 20 additional buds I didn't want to turn into additional trunks.

I'll probably keep 3 or 5 trunks and prune off the others, eventually. For now, they provide energy that will help to heal the enormous wound where the 3-4" trunk was chopped. Unfortunately, Schefflera is not such a prolific back-budder, but you might get lucky. If you decide to chop it, start several cuttings from the severed top as insurance. It won't harm the plant to chop it; by "insurance", I meant that starting cuttings will help to ensure you have a way to attain your goal of a multi-trunked planting.

Al