I have met this problem a lot of times in the different verifications. The simplest example is with 2 interfaces as I described. Lets take some decoder RTL IP with two interfaces of different types. One is to configure registers of this IP with some settings how to decode stream and another one interface is the stream by itself (data transaction). For simplicity lets take only predictor (model) of our scoreboard without any comparators. This predictor should produce output(decoded) stream (result transaction). The problem is that the configuration transaction may occurs on the same clock cycle with the stream transaction (stream data should be processed with new configuration) or may not occurs at all (in this case we decode stream with old configuration).
There is no problems if each stream transaction would have it own configuration transaction. In this case I add fifos in the predictor and wait until configuration and stream data is in fifos and process the data. But in my case stream data may occurs without any configuration transaction and the predictor does not know whether to wait for it in fifo or not.