It is rare and should only be done with full disclosure to all the companies you are working for.
Most will not like it due to possible conflicts of interest or even the fear of revealing propietary information regarding clients and claim processes. Keep in mind that anti-trust regulations strictly prohibit any hint of collusion regarding pricing or practices.
If you are good adjuster and located in a remote area where few IA Firms have offices, you are more likely to be able to handle stuff for multiple companies.
JD, RPA, PLCS
Executive General Adjuster
Kevin Hromas - JD,EGA,RPA,PLCS,WIND Umpire