Hi Kirsten,
To get the similar functionality as Split Polygons in ArcMap - which avoids having to interactively select a target:
- Select the (input) lines or polys that you want to use to split the existing polygon (target)
- Open the Split tool > By Feature tab.
- The Input Features will show the selected line or poly that will be used to do the splitting.
- Switch to the Target Features tab.
- Click the "Select all intersecting features" button.
- Click Split.
You may also benefit by turning off the editability of layers that you do not want to select and subsequently split:
In the Contents pane, switch to the List By Editing tab and uncheck layers from there: Specify which layers can be edited—ArcGIS Pro | ArcGIS Desktop
Let me know if this helps,
Scott