Although the person designing a work item form cannot explicitly set the tab order of controls on that form, a person can influence tab order if he or she understands how it is determined.
The tab order is determined automatically based on the "geography" of the controls inserted into containers such as the form. Basically, within each container, tab order is set from top to bottom, left to right. That is, a control with a top-left corner that is closer to the top of the container will occur earlier in tab order than a control with a top-left corner that is closer to the bottom of the container. For controls that have the same vertical placement of their top-left corners, the control that is further to the left of the container will occur earlier in tab order than a control further to the right of the container.
That same algorithm is run for each container such as the form itself, group boxes on the form, or even group boxes nested within other group boxes. So, if you want to simulate two columns where the tab order moves down the left column then moves down the right column, you can insert two group boxes next to each other on the form. Assuming both group boxes have the same vertical position on the form, all of the controls within the left most group box will occur earlier in tab order than all of the controls within the rightmost group box.