If you kept it at normal TC then you'd have to get pagers for everyone so they could run home from work every other Tuesday because someone spotted something. I don't think that would work.
The only way I can see an MP campaign working at all would be a coding nightmare. Play the game normally as you would in a regular campaign along with TC and all of that for each individual sub. Allow players to send radio reports back and forth (just to make it more interesting make them tap morse) whenever they want but only let people receive them when their relative time is the same as the radio message's send time. Then if 2 or more boats happen to arrive within a set range of one another kick it down to normal time so they can operate together. Or alternatively allow the first sub on the scene to control TC in the "bubble" until a minimum allowable range is reached then kick it back to normal time. That would be a royal pain in the posterior and I can think of several ways right off the bat to "break" it. But it's the only way I can think of to allow multiple subs to work together in an MP campaign environment.
The other way I could think to do MP would be to have more than one person on a single sub and then divide their duties like the regular AI officers you get. Weapons officer would handle that but not have access to the Radio, Nav would handle that but not have access to torpedos, etc. The Captain would have repeaters for everything but if a station is manned then the officer in charge of that station has to do all of the control inputs, and the Captain would have the TC key. The one station I'd let anyone man would be the guns, because guns are just fun.
Unfortunately I don't see any real way to do an MP campaign that wouldn't be more complex to code than the benefit you'd get from it. At least not realtime, if there were a program that would collect and report data from multiple log files in realtime to make it "seem" like everyone's out there in the Pacific together then that would be cool, but you probably would never be able to run into a buddy's boat and run alongside flashing signal lamps at one another.
In flight sims you can do coop campaigns because the time frame is usually all in the matter of a day at most, with subs you're talking months for a long mission.