i wanna do portamento between two notes

but there's also another note below them.....which i don't want it to be affected

do i need to separate it from the same midi?

is there a way to keep it in same midi and do that?

 

First of all: the synth you are using needs to have a portamento function. The easiest way in Reaper to do portamento is just to overlap notes a tiny bit, but having several notes playing at the same time will probably mess this up.

Since you are saying that there are several notes playing at the same time I'm guessing it's a polyphonic synth/patch or sample player? It's difficult to get this to work like you want it to since just enabling slide/portamento will make the instrument slide between all notes (as long as they are connected or overlapping). It probably works with MPE, but I don't know much about it so there are probably others here who can help you with that better than I can.

 

It's not related to your DAW, but to the synth you're using. So Reaper have nothing to do with it.

 

look into Legato/force Legato, and pitch bend automation for whatever synth using CC info. You are almost trying to do an 808 glide, without an 808 or a glide? There are lots of way to do this and you should look into all of them. With CC automation, when you move the midi to another channel/synth; it will go along with it. (your daw determines wether you copy the automation data or not by default, but you can usually change this copy/paste/move behavior in your preferences).