I have a Splunk sandbox with a certain amount of licensing. A separate BU has their own splunk instance, and was wondering if I could "share" some licensing with them. Is this possible(pools, stacks, master/slave?), or do I need to contact the licensing department to create two separate license files?
Assuming you are using 4.2x or greater, you are able to legitimately 'share' your license with any other BU within your organization - just be careful not to exceed your license volume in doing so. Feel free to reach out to your splunk account manager if you have additional questions; if you don't know who your rep is you can email firstname.lastname@example.org and the appropriate person will follow up with you.
No need to contact sales - you can do it through your existing license. Check out the Splunk Admin Manual - Manage Splunk licenses for some guidance: http://docs.splunk.com/Documentation/Splunk/5.0/Admin/Configurealicensemaster
If you are an Enterprise customer, you can use groups/stacks/pools etc. Read Groups, stacks, pools, and other terminology in the Admin Manual for more information. The topics that follow that one explain how configure masters and slaves, create and edit pools, and more.