Building a yeast starter

I'm building a yeast starter for a weekend batch. I've currently got a quart going (started many days ago and has since settled). I'd like to add another quart to both increase yeast and get it active again. My question is: should I add a quart of 'wort' to the existing quart, or siphon off the existing wort (preserving the yeast at the bottom) and add the new wort? Is there any problem with the latter approach? (I'd prefer the latter due to space constraints in my starter jar.)

If your current starter has finished & settled out, i would decant the spent "beer" and add the fresh wort on top of the mini-cake. Alternatively, i would add the fresh wort to the current starter WITHOUT decanting if the current starter was active and working.