FAR Subpart 16.7 addresses Agreements, and BPAs are addressed at FAR 13.303.  Addtional information can be found at DFARS 216.7 and the DFARS Procedures Guidance and Instructions (PGI) Subpart 216.7.  Each BPA is governed by the terms and conditions contained in the agreement, which should include identification of the authorized or supported agencies and ordering procedures.  If mutliple agencies desired to consolidate their needs into a single BPA, or modify their existing BPAs, they would mutually agree on the adminstration process and related details.