I am frequently asked about being able to simulate multple prime mover sound effects from a single decoder as well as being able to simulate dual prime mover effects typical of an EMD E8 or DDA40X.
The TSU750 has an equalizer/reverb effect built into the circuitry. Adjusting the reverb can simulate the effect of two diesels convincingly.
I think there is a very noticable effect provided by placing two or more sound-equipped diesels together on a layout. The effect is becomes distinctly identifiable when the "rummble" gets out out of phase and you suddenly realize there are tw diesels running and you go "oh, that's cool". I did an E6 A-B set with a pair of TSU750's and the reverb trick to simulate the fact that the E units have two engines per body. The effect was awesome.
Now, simulating more than one prime mover from a loco or sound car... Other than reverb and that noted above, there is not much I can do. Multiple speakers help volume but reproduce the exact same sound. It would require multiple decoders and at some point it may be better to put them right in the loco.
The other limitation to consider is the programmed sound file.
Digitrax has a combination file with a steam/diesel mix that allows you to select the soundset by changing a CV value. It is also programmable but the sound sets are limited and not all are of the highest quality.
Soundtraxx has a fixed sound set meaning you get what get. You can change the horn but that about it.
ESU LokSound is programmable and has a good selection of sound sets and are of good quality. They do not have a dual sound set though. It may be possible to create a dual prime mover sound file on the ESU platform but I have never investigated the possibility.
The Digi and ESU do require a programmer to change the sound files. Not a big deal but a required tool.
