Going from WO to ASSIGNMENT isn't possible in the start center portlets today. We only support 1:1 relationships like going from a WO to ASSET based on the ASSETNUM field. By default, we prevent customers from selecting data from a 1:many relationship by the webclient.allowMultipleCardinality system property being set to 0 (the default) to avoid confusion.
What you should be able to do is the reverse (going from ASSIGNMENT to WO) but it's a bit tricky because there is no application out of the box you can use. A result set portlet must have an application where the main object is the main object of that application. Assignment Manager isn't available to be selected for portlets (it's a special type of app) and that's the only one where the main object is set to ASSIGNMENT. But if you create a new application where ASSIGNMENT is the main object and then create a Report Object Structure (ROS) that has ASSIGNMENT and WORKORDER you would be able to display data from both.
Something to be aware of is that you can filter on related attributes, but you cannot sort on them. If you needed to sort based on Work Order scheduled finish for example, that would be a problem with ASSIGNMENT being the main object.
As we continue to enhance our new dashboards, we'll hopefully soon have a much better answer here.