Not many adhesives of any sort stick to their dried or cured brethren. That's why you should always clean them off, or at least give the next best thing which is a good cleaning and/or roughing up.

In the case of silicone adhesive, I'd just make sure it's clean. Maybe wipe it with some alcohol. Possibly rubbing a little bit of the new silicone on the old prior to laying your bead of silicone will help ensure a good bond and that your bead is not just laying on top the old.

But......... I've never liked the idea of dividers for separating water and dry... if that's what you are using it for.
It is likely to eventually leak from what I've read. And when it does, you will have a mess on what should have been your dry side. Which will require a complete redo. It also prevents any live plants on the dry side from helping to maintain your water quality. Plants can remove the ammonia and some other "bad" stuff if their roots are allowed to get down to the water.