Another little know fact. Once you make any changes (during the mixdown) to your 16 bit files - adjusting EQ, adding reverb, etc, the wordlength gets larger to match the operating resolution of the DAW. That's why you should always mix down to 24bit (or 32 bit float) even when working with 16 bit tracks.
funny, i was just thinking about this yesterday. do you even have to make any changes? if the DAW soft has a native processing resolution, doesn't the wordlength get larger to match the operating resolution of the DAW by simply running it through the mixer and subsequently exporting, rendering, bouncing or mixing down?
anyway, until Mark shows...
in this situation, the only benefit i can see to having two machines, would be if the AD and DA converters on both machines are really good and you prefer the AD/DA conversion to your softs (or plugs) dithering process/quality.
internal vs. external bounce: no need for two machines... you can simply export/bounce/mixdown/render... (lol etc.) to a 24 or 32 bit wav file, then create a new project/session to master that file. couple of benefits over the internal mixdown you're doing now...
- more clearly seperates the mixing and mastering processes. keeps the workflow cleaner.
- helps you avoid double dipping of the master fx. if you mixdown, to a track within the session, without turning off the Master fx, when you later export to wav/mp3, it will go through those Master fx a 2nd time.
- you can free up any cpu/ram that the multitrack session was using.
anyway, as Cary said (or implied), your limitation is self imposed. whatever work method you choose, internal or external (to the session not the machine), mix to 24bit (or 32 bit float)
general note about dither... most DAW softs have their own dithering process and it's usually enabled by default. check to see if it's on and which method it's using. if it is on, there's no need to insert a 2nd dither plugin on the Master.