How do you break up responsibilities when you're the first and only PMM and now you are hiring a new PMM to work under you? In a fast growing SAAS startup?

I tend to coalesce early-ish stage PMM teams by product pillar/functionality. For example, if you group certain features into capabilities, split roadmap ownership betwee you and your PMM. Bucket those product capabilities and divide and conquer (full PMM GTM lifecycle responsibilities). Once you've achieved product market fit, you can consider organizing the team by relevant personas and begin to hire specializations like: core PMM, customer marketing, lifecycle marketing, competitive intel, etc.

But definitely index on hiring generalists that have a platforms & systems mindset. These folks can scale and that's what you need when it's a small but mighty team. 

Congrats on growing the team! In a two person team, you have to look at the top priorities, where you can have the biggest impact (where you are uniquely needed the most), and where each of you are strong and excited to grow. You are in the fortunate position to consider what elements of PMM you enjoy most and which ones you would be happy to delegate so don't forget it. 

If you're really not sure, try doing a quick exercise of listing out all the PMM functions you support today, new functions you may need to support in the coming months/year, and rate them by size, impact, priority, and list out internal stakeholders/collaborators. From here you can start to see patterns and how it might make sense to break up work.