There is a 60 day grace period should you be laid off and lose your sponsorship. That being said if you need more time to set things up properly for your new startup to (a) exist and (b) sponsor your cofounder’s immigration, your cofounder can apply for a change of status to B visitor. As a B-1 business visitor, your cofounder can engage in certain activities legally, such as business formation and fundraising meetings, and request an additional 6 months of time runway beyond the 60-day grace period. This application process can run in parallel with immigration sponsorship by a new company.