Both companies are from Beijing. How do they transfer insurance from one company to another?

According to the provisions of Article 3 of the State (Guo Ban Fa [2009] No.66), if the insured person is employed across provinces, the social insurance agency of the original insured place (hereinafter referred to as the social insurance agency) will issue the insurance payment certificate, and the basic old-age insurance relationship should be transferred to the new insured place. If the insured reaches the conditions for receiving the basic old-age insurance benefits, the payment period of insurance premiums in various places shall be calculated together, and the amount of personal account storage (including principal and interest, the same below) shall be calculated cumulatively.

Therefore, from one company to another, whether in or outside the province, the insurance relationship can continue to take effect, and the payment period can be calculated together.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.