No ISP will provide internet transit to company B using AS of company A.
If any ISP provide internet transit to company B using AS of company A, lawyers of company A will be knocking at company B door the next day by using their AS unauthorized and the ISP that provide transit may also get into trouble
Technically it is possible to use one AS in two routers without backend connection provided that the IP range are different. Here is the link to the discussion about this https://supportforums.cisco.com/thread/208814
Please check the link to the discussion I provided. The scenario is similar.
The only difference is that the scenario in the link I provided is that the AS remains in one company but have two sites not connected. Your scenario is that the AS will be use by two companies.