In my experience grafting cannabis has been more successful if you graft during the cotyledon (aka seedling) stage.If your grafting this early it would be best to plant in the same container staggering your germination to allow the stock to sprout first, then after the scion pops up let it grow for a day or two, with careful surgeon like precision you will need to slice an oh so thin layer of tissue from the stems of both plants, combine the two cuts and tie of with a rubber band or other suitable grafting material. After the graft takes cut the stem below the scion. Prune to encourage an open structure and voila you now have two strains on one mother, just remeber to label the branches.Its also possible to do this durring the early vegging stage. This gives the graft a good amount of time to callus. Its best to grow your stock and scion closesly together, give the stock about 3 days head start then germinate the scion, they need to be pretty close in thickness. Next you would need to take your stock and cut a thin sliver out of the side vertically, only enough to expose the cambium(inner tissue behind bark) repeat for the scion, now heres where it gets tricky. You might have to ghetto rig something to keep the two plant pots together after you tie off your grafts. Other than this I know of no other way to really graft cannabis. You could sustain your new multi-mum by bonsai techniques and keeping it in permanant veg. Ive read about people keepin bonsai moms for over 5-10 years! Good Luck and remember BE CLEAN ABOUT IT!
*NOTE if you dont know what a stock and scion is read this example:
Say I wanted to use a blueberry as my main mere. But on that blueberry I also wanted a bubblegum. The blueberry would serve as the stock and the bubblegum would be a scion. If its still muddy ask a question