The YASSD is controlled via a UART connection, via a couple of simple commands, at a baudrate of 19200:
To control what is displayed:

ID,NUMBER\n

So if the display has an ID of 04 and you want to display 1337:

04,1337\n

A dot is no problem, just insert it whereever:

04,1.234\n   or 04,123.4\n

To change the brightness:

ID,Bx, with x being 0 to F (hexadecimal 0-15) for the brightness, 0 being the lowest, F the highest, for example:

04,B2\n  or 04,BA\n

As long as YASSD boards have a different ID set by the solder jumbers, they can be connected to the same UART bus, a bit like how I2C works.