You can cut the yarn where it's joined, but you won't have long enough tails to weave them in. You'll probably have to rip out to the join, which isn't that difficult and less time than it takes to rip and start over. There are videos on the Tips page under Fixing mistakes that show you how to put your needle in. Rip back to where you joined them, then insert your needle into the sts as you pull out the sts on that last row. If you do it one by one, inserting the needle as you pull on the yarn, it's easy to do, and you won't drop any sts as long as you don't stretch out the piece.