Yes, this is correct. New combined license key automatically use the earliest support expiration date for future renewals. Here is an example below:
Contract #1 for 1 Backup STD with support till 25/3/2015
Contract #2 for 2 Backup STD with support till 12/6/2014
Merged file for Contracts #1 and #2 = 3 Backup STD with support till 12/6/2014.
That mean you can ask for Veeam support for merged license file (as well as for contract #2) only till 12/6/2014 at the same time you can ask for Veeam support for contract #1 (single license file for 1 Backup STD) till 25/3/2015.