You can experiment around with this with a
SoftSqueeze.
As soon as there are two Squeezeboxes attached to
SlimServer, an option to Synchronize will appear
in the menus of each player.
Select it and it will say Synchronize with and
the name of the other player.
I haven't experimented around
If you don't want any audio from the other player, you might be better
off with the ShadowPlay plugin,
Which mirrors the interface from another player.
Then SlimServer's not doing the work associated with syncronising
players.
That would be somewhat of a waste of a SB3 though!
James