Hello,
on DNN creative I am using two separate house menus. - to create a side vertical menu for each of the main horizontal menu items you have to create a separate skin for each vertical menu (in the housemenu) - whereas the Snapsis menu can be setup much more easily in the menus configuration options.
If you want to learn how I did this in the house menu have a look at this tutorial:
How to create Horizontal CSS menus in DotNetNuke using the HouseMenuthis is for creating multiple horizontal menus, but the same principles will apply where one menu is horizontal and the other is vertical.
If I was approaching this again I would create this type of menu system using the Snapsis menu as it is much quicker to set up
hope this helps,
Thanks,