I requested this 5 years ago... but I understand the separation of L1 and L2 feeds is probably important and convenient organization of the feed data.
However, can we request the messages to contain a new field, message sequence ID, that comes directly from the exchange for both L1 and L2 update messages? This would allow the client to synchronize on their end. And you must be already parsing those from say CME in your data processing anyway. You could just pass them along (as optional field) to the client.
and tag 83 message field described as:
FIX Type: Int
Description: MD Entry sequence number per instrument update. Reset weekly.