What is the main goal behind the multiple LLCs: to keep separate books for your business vs the whole, or liability protection from the other partner?

TIP Getting into business with anyone - family, friends, stranger - should not be done so without an Operating Agreement (OA). OAs offer protection to each partner, and outlines what actions the partner or partnership can do in any situation, i.e. do you both have to approve large purchases over $5k, what if a member’s capital account is negative, who is the managing member?

Edit to add tip.

E commerce has a host of issues itself. Although helpful in the short term, there are state sales tax nexus issues that would need to be addressed and can take time/money to do correctly. (US based store assumed, SOUTH DAKOTA v. WAYFAIR, INC., ET AL. 2018).