duple vs triple

This should work,

Are you trying to do this with linked staves. They are intended to show the same music in different formats - e.g. standard stave on one and TAB on another. Thus, you can't have different notes in two linked staves.
If you want independent staves you should just add an ordinary stave - see https://musescore.org/en/handbook/4/working-instruments#add-configure-s…

[Edit] or start by selecting piano as the instrument when you first set up the score - it will have two unlinked staves.

If that is not the source of your difficulty please provide some more information:

• Which version of Musescore are you using and which operating system? You can go to Help>About and press the copy button there and paste the result into your post.
• What steps are you taking to enter the eighths and triplets and what exactly goes wrong and at what step?
• Do you see an error message? If so what does it say?
• It may help if you attach the score (.mscz file) that you are having difficulty with

A triplet is just one kind of tuplet - so search in the Handbook for creating tuplets. https://musescore.org/en/handbook/3/tuplets (it works the same way in MS4)

There is no problem to create triplets, even in the same staff if you use another voice.