I'm doing research for a project I'm asking for bids on and I'm wondering how much you can mix decorative concrete treatments. I want a large exposed aggregate patio, but in order to break up this monolithic slab, i want to add a border. I have seen plenty of examples of a stamped border around exposed aggregate but I'm wondering how it was done. Is it done with separate pours - one for the exposed agg, and then another for the stamped border? Or is it more generally done with a single pours, stamping the border and then taking care not to hit that border with the retarder used to stop the cement from hardening? If the latter what care must be taken. In the former (two pours), what is typically done to lock together the two sections and/or why are two pours preferred?