Back in the SH1 days they offered two strart locations: Pearl Harbor and Manila. This led me to start two side-by-side careers, one from each base. As other bases were added I started new parallel careers. If one retired or died I started a new one at that same month.
Today I have three careers going in SH3, one from each flotilla. In SH4 I have the same thing at the same time, all in u-boats. If I ever actually get to 1942 I'll be running fleet boat careers as well, all simultaneous. Someday I might actually reach the end of the war.