If you have enough points without borrowing, book a 1 BR and a 2 BR for the entire stay. If you don't have enough points, next option would be to book the 1 BR & 2 BR for weeknights only. If you'll have to borrow points to do the 2 BR period, I'd simply book the 1 BR for your stay then try to change any dates you need to a 2 BR when you know the dates. Of course you could get your 1 BR then add a studio for them for the days you need. You could eve ask they be separate but connected if you want. A few more points than simply getting the 2 BR but a guarantee of not changing units and a more flexible reservation if they change their mind in any way.
As noted, you can't simply add a studio to a 1 BR nor can you simply drop a studio from a 2 BR to make it a 1 BR. And no way to guarantee you'll be in the same unit for the 1 BR vs 2 BR portions. Thus if you had a 1 BR reservation with a 2 BR for a few days in the middle, it's possible you could have to change units two times.