Hi,
I guess real multicore MBed OS is not currently planned, see this comment Mbed OS - New Features - #18 by MultipleMonomials
One way around this limitation would be to have two applications, one for each core. This would potentially require some synchronization between the cores, and reserving RAM, etc.
I think I saw some project in the forum here going this route, but currently I cannot find it.
Best,
Chris