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You cannot work on one tax return on two computers at the same time. If you are using the CD/Download TurboTax software the tax return is stored on the computer you are using. It cannot be accessed by another computer.
It is not recommended ... however if you want to use the PC to work the return then the MAC to continue it then back to the PC to finish it then you can ... you can save the .taxfile to a thumb drive to move it back and forth ... be very careful to save and use the most up to date file each time.