Sorry, not possible to have 2 computer voices installed at the same time. Your model (as with most recent) doesn't have a sd slot for extra expansion.
The only suggestion I can make is this .....
Call CS at 866-486-6866 and see if they will link a US map only to your account. Explain why you want to do it. If they agree AND offer up the current USA map, then it is 160 megs smaller than the one you have. That will allow for a second computer voice, I guess.
By the way, new 865 maps are due out in a few days......
If they do agree, make sure you do an Explorer, not Home, backup of your unit's current contents. Just in case.