Help sending MIDI to multiple independent VST (no DAW)

VST virtual instruments usually require a DAW to host them, although there are other apps dedicated to hosting/playing VST or other audio plugin types without (outside of) a DAW.

You might have meant standalone apps, that work outside of a DAW with no host whatsoever. Pianoteq includes both an audio plugin (to use inside a DAW) and a standalone Pianoteq app. Many other apps have standalone versions (and often include plugin versions as well), including Pianoteq, Organteq, Decent Sampler, Dexed (a Yamaha DX7 clone/reconstruction), Surge synthesizer, etc. (the list is a long on).

You didn't mention what OS you're using. I use Linux (and know very little about Windows or Mac OSX).

In my Linux computer OS (Linux Mint, XFCE edition), my digital piano MIDI keyboard is connected directly to the computer using a USB cable (I sometimes use a MIDI hub or external audio interface), where the computer's audio systems label it "USB-MIDI MIDI 1"

I'm playing three standalone audio apps (Pianoteq, Decent Sampler & Dexed) at the same time (refer to the image below). I have opened all three apps' Audio-MIDI options/settings and have chosen "USB-MIDI MIDI 1" as the source of MIDI data to respond to. So all three apps receive the MIDI and all three apps produce audio output, which is routed to the headphone/speaker output of the computer's internal hardware audio interface included on the motherboard.

In DAWs or in non-DAW VST-hosting apps, one usually specifies and configures one or more MIDI inputs to control all of the VST or other plugins that can receive MIDI input. But in standalone apps, one must either: specify the MIDI source in the settings of each app, or use a separate routing app like JACK, with which one can route MIDI and audio sources/outputs to MIDI and audio inputs of other apps or to the MIDI and audio ports of attached hardware including audio and/or MIDI interfaces.

Audio streams
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An important thing to remember, I think, is that if you use multiple audio apps outside of a DAW, they are each producing their own, independent audio stream (regardless of how many channels that audio stream has: 2-channel stereo, 5-channel, etc.) and because of that the audio output of each app should be directed to a computer audio-system and mixer that can manage multiple audio streams, and merge them in order to send them to the input of a single audio card.

In a DAW, each track may produce its own audio stream, but the DAW itself merges all of those individual audio streams into a single (stereo or multichannel) output stream, to send to a computer audio card or external hardware audio interface. So a DAW merges the audio internally, so that the mixture of all of them together, in a single audio stream, are sent to the sound card / audio interface.

When you have multiple apps outside a DAW, each one of them is competing for the input of an audio card; so if one app is first to "claim" the audio card it may monopolize it, and other apps may not be able to access that sound/audio card input. That's why you probably will need an "audio merging" app when you use audio apps like Pianoteq, Organteq and others. That "audio merging" app is (or can be) typically the OS system's default audio mixer, which can receive input from several audio apps (and a web browser and other apps that happen to produce some audio content), and then merge them into just one audio stream to send to the headphone/speaker audio port of the computer or (for example) via USB to an external audio interface, to play back either just one audio source, or several or many audio sources merged into just one audio stream. In Linux, that system audio mixer is often Pulse Audio, but Pipewire and JACK are also used as system or special-purpose audio mixers (audio stream mergers with volume controls).

In the image below, one can see that the audio output of all three MIDI/sound apps has been directed to the PulseAudio server; that's why I can hear all three apps' audio output streams at the same time: because they are merged into just one stream by the PulseAudio system via the PulseAudio graphical mixer before being sent to the desired audio output port (the headphone/speaker port of the computer).

To see the image full-size, click this link or right-click-->Open Image in New Tab" in the web browser.

https://i.imgur.com/sLz5Kiv.jpg