Run the wizard (Wizards | Bes Support | File Pre-Cache Wizard)
Select a fixlet or baseline you are intending to send to your clients using those remote relays
That will create an action - you can set the targets (your two remote relays) and execution details, so the network impact is at a time of your choosing.
The action just runs all the downloads for your fixlet/baseline. It does nothing with those files, just downloads them, but because it is run on the relay, the relay cache will then have the files.