upgrade int'l S-class ticket using miles from two accounts?

I want to buy a ticket for my sister to visit me (roundtrip IAD to BAH). The flight is long and so she would like to upgrade using miles. When I look at booking the ticket on United.com, I can do so for the cost of the S-class ticket plus 35000 miles & $600 each way. The issue is, we want to pull 35000 miles from her account and 35000 from mine.

I know I can buy the ticket and only upgrade one way at that time. My question is will she then be able to log into her account and upgrade the other half of the ticket for the same 35000 miles & $600? Or is that option only open when initially booking the ticket?

I am United Gold, she is United Silver if that makes any difference.

Thanks for the help!

edited to add:
alternatively, might I be better off just paying for Z-class?

S-class ticket costs $1334
upgrade costs $1200 plus 70,000 miles
Z-class ticket costs $3995

If I get the Z-class ticket, I'm essentially playing $1461 to keep 70,000 miles. Buying 70,000 miles from United costs $2633.75 so I'm not sure if I'm better off paying for the Z-class ticket in this case?