Increasing when joining 2 legs and staying in pattern
It's the first time I have knit anything since my son was born 30 years ago and I have got a pattern to knit an all in one baby suit. The pattern is King Cole 3504 Aran. I have knit both legs following the pattern and have even managed the cable!! But I have been instructed to join the legs together by increasing 1 stitch and the end of the first leg then adding then second leg from the other needle the instruction then tells to to continue in pattern for another 15 rows but I know that the pattern will be incorrect and will run off. The cable runs down the side but the patterning in the middle will surely run off as I have no idea what to do with the extra stitch. Can anyone help please?
Thanks in advance for any advice.