You can if you push the majority of your logic into PCL libraries. I've done quite a few cross platform apps that share codebases with WPF, Silverlight, Winforms, WinRT etc. You will of course need separate projects for the UI side of things.
If you install now it is showing the Universal SDK as being available. If you've already installed VS2015, Find it in programs and features and select modify. The new bits and pieces will show up (With new in brackets next to them even!).