Hello artidth,
Serial numbers with different Subscription end dates cannot be merged, indeed. The customer would need to order a Subscription extension to change the end date on one of the serial numbers.
However, it seems that you're having a different problem: you cannot see all the products of your customer. The reason for that could be that you are not assigned as the supporting reseller on all of the customer's serial numbers. In that case, provided that you are an authorized reseller for the products you want to see, you would need to ask the customer to fill & sign the 'Change of Reseller' form, which you would then send to the Autodesk Business Center through Partner Center. They would then assign your company as the supporting reseller on those serial numbers and you would see those products.
Finally, you also mentioned that the customer has 2 accounts. Are you referring to user accounts (i.e.: the user has 2 different User ID's for Subscription Center), or are you referring to the company accounts (i.e.: the company has 2 different CSN = Customer Specific Number)? In both cases, for visibility purposes, it would be better to get rid of duplicates. The Autodesk Business Center can help you with this.
Regards.

Simona Golisciano