I started my first tech startup with a friend where he had the initial idea and then I joined him to develop the same further. So I understand what you are referring to.
Since you have a team of 4 - I am wondering why are you only talking about the the other co-founder. I assume here the roles & responsibilities of the 2 techies are clear to all 4 involved.
Its always a good idea to have a clearly defined set of tasks between 2 co-founders. To define the same you need to look at what needs to get done and who is best suited to do it. Some areas like product demos/ reaching out to contacts etc can be shared. However the core responsibilities should be clear in terms of what you & the other person will "get done"
This is necessarily a collaborative & exploratory process between the 2 of you or 4 of you. If you encounter trouble on this path - its best taken head on & addressed as those issues could or rather will come back to bite you hard.
I hope this helps. In case you need any further specific intervention please do let me know. All the best.