You should showcase your experience but brevity and clarity are key. I tend toward "non-material truth altering" in this case. What I mean is not to lie, but standardize the presentation to the industry. You can explain any detail that might matter in an interview but you don't want to raise confusion for someone scanning your resume.
Unless Company B has some clout/name recognition that's important to include, I'd omit it.
Project Manager / Sales Manager (2015 - Present) Company A
- item 1
- item 2
If it should be named, you could add it in one of the bullets:
- Managed sales activity for 50 accounts for subsidiary Company B
Edit A third consideration would be that you should tailor your resume to the job to which you're applying. If you're applying for a PM job, drop "Sales Manager" from the job title but call it out as an accomplishment in the bullets if of value.
Listing them as you laid out would lead me to first wonder if the matching dates were in error. If I hadn't already tossed the resume, my next question would be are these two start-ups and neither role is really a "serious" (for lack of a better term) and you're just attempting to make it sound more professional?
The upshot is you only have seconds to tell your story. Anything that is not immediately clear is dangerous. Your resume/CV is to demonstrate the valuable experience you could apply to an employer. It is not to train them in the intricacies of how your current employer is structured. So long as you are depicting your experience in good faith, the odd nuances of your specific situation should be held for when you actually have time to explain.