There are a few options to do what you're after... Here are some snippets from my config files, that I use to open certain applications in certain workspaces, on certain monitors.. Hopefully this makes sense, if not, feel free to ask any questions, and I'll do my best to answer. The only bit I'm not certain about is automatically changing from a horizontal split to a vertical split automatically. I assume that you can do this by sending an "i3-msg split h|v" command between launching the programs.
examples from my config file....
# Workspace Names
set $ws3 "1: Terminals "
set $ws3 "3: Web "
# Set up monitor orientation (my i3 starts with these reversed, so the following command swaps monitor positions)
exec xrandr --output DFP11 --auto --right-of DFP10
# Place Output on proper monitors (every time you initiate a new ws, it will start automatically on whichever output...)
workspace $ws1 output DFP10
workspace $ws2 output DFP10
workspace $ws3 output DFP11
# set default workspaces for specific applications
assign [class="google-chrome"] $ws3
# start up applications
exec --no-startup-id i3-msg 'workspace $ws2; exec thunar; exec thunar'
exec --no-startup-id thunderbird
exec --no-startup-id subl
# next line will start up two terminals, perhaps you can add an "; exec i3-msg split v" between terminals?
exec --no-startup-id i3-msg 'workspace $ws1; exec urxvt; exec gnome-terminal; workspace $ws1'
exec --no-startup-id i3-msg 'workspace $ws3; exec --no-startup-id google-chrome-stable; workspace $ws3'
# send specific workspaces to specific monitors.
# in the example above, the below commands aren't necessary because I start the applications
# in the workspace I want, which is assigned to a certain monitor. If you didn't set all of that up,
# you could use the following to send workspaces to whichever monitor you want.
#exec --no-startup-id i3-msg 'workspace $ws1; move workspace to output left'
#exec --no-startup-id i3-msg 'workspace $ws3; move workspace to output right'