You have not specified which split you mean. I can see several trunk forks in the pics.
Nor have you offered any view of what style you would like to develop after the chop.
It may be quick to post just a few words but lack of info makes it muc harder to offer meaningful advice.
This appears to be a Chinese elm. the lower trunk is vertical and splits into several sub trunks. Both those attributes suggest broom style to me.
I can't properly assess a tree's 3D possibilities but I think pruning above the first and second forks would probably give you the basis for a reasonable broom style Chinese elm bonsai.
Cuts marked in red here
I've cut the larger trunk a little lower than the smaller left ones to allow for the new branches growing from the larger side to grow a bit before more cuts to add ramification. the fork on the left side is just about right for broom style ramification so prune above that fork.
If you were planning on another different style let us know so people can offer informed suggestions.