Simultaneous sound output for both Pianoteq software & PC apps

Assuming this is Windows (because you mentioned VSTs) — no promises this will work, but try:

First, take a look in the Pianoteq standalone at Options > Devices and see what is in “Audio device type.” If it says “Windows Audio (Exclusive Mode), that’s the problem. If “ASIO” is an option, use that. Otherwise, try “Windows Audio (Low Latency Mode).”

If the above doesn’t fix it, take a look at Start menu > Settings (the gear on the left) > System > Sound > Device Properties > Additional device properties > Advanced and see if changing “Allow applications to take exclusive control of this device” makes any difference.

Not all audio interfaces support this. You'll want to look for an interface that is fully "multi-client" capable. Unfortunately many of the audio interfaces that do support it, don't advertise it. You can however find lists online. I believe NI interfaces all support it, as do a few other brands I don't remember off the top of my head... MOTU and RME I think?

This is one of the reasons I ended up moving to Linux for audio specific use. There is a JACK-like program available for Windows, which isn't free, and which I tried briefly years back, but it didn't solve all my issues. It might work for what you need if you want to avoid a good audio interface. The company that makes it has several programs, and now I can't remember which is which: https://vb-audio.com/index.htm

Hello all,
Thanks for the replies and suggestions. I was able to play around and learned some things.

- I was able to fix my audio issue with Coises' suggestion by changing the "Audio Device Type" from ASIO (ASIO4ALL v2) to Windows low latency. One thing I'm noticing when testing between the two device types however is that there is a very small touch of lag introduced in the Windows Low Latency Mode. I could be imagining this and it certainly doesn't impact what I'm trying to do, especially at my skill level.

- I also appreciate the other suggestions regarding JACK and it got me thinking; Perhaps my original intention of buying a new sound card would eventually have me end up needing a mixer (digital or physical) in the end? I was looking to have all of the sounds to one output, my headphones (or desk speakers), and from what I'm gathering here, I would need a mixer to join the output of the two sound cards into the input of my headphones, much like the diagram Stephen_Doonan had posted of their JACK setup.
Perhaps a follow-up for my own curiosity: Does adding in mixers and what not introduce noticeable delay/lag? I know nothing of audio production and the workings behind it but playing around with the Audio Buffer sizes in PianoTeq made me realize that this can be a huge rabbit hole when equipment starts to expand.

Thanks all!

You are probably not imagining it. I find it quite annoying to use anything but ASIO. That little bit of lag (called “latency”) is still enough to be disconcerting.

Rather that try to use two audio interfaces at once (I haven’t tried that, but I would hardly be surprised if it causes new problems), get a single interface that supports ASIO with its own drivers (rather than needing ASIO4ALL) and permits multiple streams at once. The one I have — the Roland Quad-Capture — has no problem with this. It’s been discontinued, but I suspect many, if not most professional interfaces can do this, too. It does matter, though, to have an interface that’s designed for musicians and recording engineers, rather than one meant for typical consumers or gamers. (I have no personal experience with them, but a lot of people seem to like the Focusrite Scarlett line. People love RME interfaces, but they are quite expensive.)

Most folks use a USB audio interface rather than a sound card these days. Not that there’s anything wrong with sound cards in principle, but there are few now made for musicians aside from some very expensive gear.