How do we add a set of antennas for our Tempest system?  Can you let us know what is best practice for combining these antennas? Do you sell the hardware?
What you need is an antenna multiplexer.  We can’t sell that sort of device for Tempest because when the FCC certifies the product, it does so from one end to another. Adding the multiplexer would put us out of compliance. An after-market supplier can help with this solution.

A company who can help you is ProductionWireless in Burbank, CA, USA.Contact: Greg Parsons (the owner) (818) 840-9311
 Clear-Com do not sell Tempest anymore, though we continue to support the product. If you need to buy batteries, or other things for theTempest you have, you can also get them from Production Wireless, who is anauthorised dealer.
NB: We used to recommend Channel D Solutions, in Nashville, TN, for such solutions but they went out of business and are not available anymore.