The easist is if you have a multi track that you can record into... As you can let the caustic song go on one track and you record from heat synth unto a seperate track and then cut out the parts you like.
But Heat synth also have the possiblity to record directly in the application. So even if you do not have a multi track but have at least some way to hear the Caustic track you are working on you can record the things in the application and import it into Caustic. Also as Heat synth has a Sequencer / arpeggiator it is nice to set the BPM and try things out and export / import...
So in thsi way you could use Caustic as the multi track with samplers and build up whole songs using just Heat Synth.
With Caustic 3 it is nice that you can edit the sounds directly in Caustic making it even easier!
This is the time the night and the year
please I don't understand
You need to give us more information if you want us to help you. "please I don't understand" doesn't help ... WHAT don't you understand? WHAT are you trying to do exactly?
Ulltimately they're separate apps and were never meant to work together. You'll need to put in a bit more effort than this if you want to make it work.
based on a quick internet search i gather that heat synth is an app that acts as a vst plug in for certain sequencing music programs. to my knowledge caustic does not support vst plug ins.
so to answer the original question. you cant. but if heatsynth allows you to record sounds and export them as wav files you can do that and then add them as presets in the pcmsynth. or drumkit samples in the beatbox.
hope i dont look like a fool trying to answer this but its worth an attempt.
My two cents then - you can't use a Vst plug-in in Caustic - Caustic doesn't support Vst technology. Therefore you can't directly use Heat Synth from within Caustic, nor can you use Caustic from within Heat Synth.
You can still potentially use the two apps together but at one step removed - i.e Sampling and/or midi export
1. You can export audio and/or midi data created in Caustic into another DAW/Multi track recorder, if this DAW supports Vst technology then you can use Heat Synth natively from within your DAW
and if Heat Synth allows you to save files/patches/sequences as audio files, preferably in .wav format then...
2. You can also export audio created with Heat Synth (from Heat Synth itself or from another DAW if you're using Heat Synth as a plug-in) as .wav files and use the PCM Synth in Caustic as a sampling keyboard to play and manipulate the audio created with Heat Synth and then add more Caustic machines (synths) to create a tune
(the danger with 'simple' explanations is that they become very convoluted, very quickly)