I think you’ll be OK. Even if you’ve made a double sulfur dioxide addition to your Chambourcin grape must it should eventually still take off, albeit perhaps a bit more slowly than you expected. Let’s say your normal dose was to add 30 ppm to the must and you added 60 ppm instead. Thankfully, during a must or juice addition, a lot of the free antibacterial SO2 gets bound up very quickly and so after a short period of time (like maybe 6 hours or so) won’t even be available anymore to inhibit yeast you might pitch. Also, yeast are less sensitive to sulfur dioxide than bacteria are; this is why a juice or must SO2 addition is mostly done anyway, to inhibit malicious bacteria, not necessarily to “kill” any yeast brought in from the vineyard.
If this is your situation, I’d say to wait at least 6 hours to pitch your yeast to give the excess SO2 a chance to bind up and be less of an issue. It might help to give your must or juice a good stir in order to distribute the sulfur dioxide completely and to encourage binding. In the meantime, you want to give your yeast the happiest possible fermentation conditions, so make sure your juice or must temperature isn’t below 60 °F (16 °C), there isn’t too much acid (keep pH above 3.35), and consider adding a dose of yeast nutrient.