To claim points for work experience you can combine work at different employers, they do not have to be the same. However, you have to meet the 1 year full time work experience requirement in the NOC you have selected.
If your case, I do not see a problem, even if you provide letters from different companies. However, to claim points you will need a reference letter that states your job duties, salary, number of hours worked as per the IRCC format.
If you cannot get a reference letter, you can get a letter an letter and have it notarized. This letter can be form a former colleague, or anyone who is aware of your work at the company and they can list the same things as would have been in the reference letter. You can then use this letter with your pay slips and explain it in LoE.