Do you trade with Interval International or through Marriott itself?
Call whoever you trade with (interval or marriott) and ask them about splitting your week. You'll have to decide to split it when you DEPOSIT, and can't really change your mind after, but I'm pretty sure Marriott owners get to "request first" so there's even less risk of deciding to split and then changing your mind or whatever.
I own a two bedroom lock-off one week every other year (not with Marriott but we trade through Interval and almost always stay at Marriotts because I like them). We've been able to vacation every year (sometimes twice in a year) by splitting the unit and/or using "short stays" instead of full weeks for vacation. For example, I split my 2010 unit and deposited it(pretty early) as a 1 bedroom full kitchen and 1 bedroom limited kitchen. We were able to trade for a 2 bedroom Unit in Orlando for spring break last year, and we're going back this summer again in a 2 bedroom. With my 2008 week, I used the studio lock off for 1 week in 2009, then the 1 bedroom as two separate "short stays" in 2010 for weekend trips with girlfriends.
For 2012 unit, I deposited as the full 2 bedroom because we want to go to the Caribean and there are fewer options and we want more space, so we wanted to have more trading power.
HTH
T.