You can, but not at the same time. The data and address bus is shared. IO is done by the 8502 (the 6502 variant) when in Z80 mode, but only one CPU is executing code at the same time - the other one is halted. It was a shame they didn't sort that out. I don't know enough about the Z80 to know how practical sharing the bus would be, the way the VIC and 6510 in the C64 does, but almost "anything" would've be better than the arrangement where either one is totally stopped.
(If you have a floppy, though, the 6502 in the disk drive would still be available, and run in parallel, but the disk drive was both severely memory constrained and of course limited to the serial bus.
The Z80 and the 6502 (well, 8502 actually) CPUs share the same bus, and so there can only be one CPU running at one point in time. However afaik in CP/M and Z80 mode there are some I/O functions for the keyboard etc that halt the Z80, switch to the 8502, do what they do, and then switch back.
I'm not sure if you can somehow programmatically switch from the Z80 to the 8502 and back, though. I suspect not.
