Currently HLAs only accept input from one analyzer. For some use-cases, taking input from multiple analyzers is necessary.
A multiple input HLA could allow for adding addons for existing analyzers. Example use case:
For SPI, adding an IO pin (often called DC for Data/Command), that emulate 9 bit Data.
If you have a parallel DAC there can be multiple signals that all constructs an analog signal.
Good idea. For example on Thunderbolt3 data from both TX and RX is important to associate commands and responses.
Serial use 2 wires with one analyzer each. You may have to group I2C & SPI for testing purposes...