As far as I’m aware there is no issue in starting a new ticket before you complete the old one, so long as you do complete the old one using all coupons/sectors in their booked sequence as originally intended. After all it’s quite plausible you needed to return to your original start point, perhaps with another airline. A new ticket is a new contract so to speak.
However, not completing the old one, leaving open unused sectors, could prompt contact from the audit or revenues department if done on a regular basis – but I get the impression you use all your sectors so it’s no problem. Perhaps if you were concerned with potential awkwardness or denied boarding you could omit your EC number for a particular booking and once the other ticket was complete submit a retrospective mileage claim for the other……. But risk forfeiting lounge access or if lucky an upgrade!!
If an EC member they are tracking us all and can easily get to know our habits……. if they have reason to take a look!