The answeres here are somewhat confusing.
The license model is chosen for the entire organization. If you have an org with co-termination, you have exactly one termination date for the whole org. Imagine you have 10 APs, license termination is in 100 days. So you have 1.000 "license days" in your org. Now you buy one single AP with a one-year license. Now you have 1.365 "license days" for 11 APs. You have to calculate 1.365/11=124. Your license for your org will terminate in 124 days.
Regarding your questions:
1) Yes. You cannot purchase other licenses but co-term licenses. But no, you will not have two co-term licenses. You have claimed two license keys, but the licenses will be combined. If the license terminates, you will buy one new license for all devices in the org.
2) No, there are not two licenses in your org, and no, co-term licenses cannot be managed by API. API managemenent is available for per-device licensing and for subscription licensing.
That's my understanding of co-term licensing. Maybe the documentation will be helpful:
https://documentation.meraki.com/General_Administration/Licensing/Meraki_Co-Termination_Licensing_Ov...
At the end of this document you will find a license calculator. Here you can enter the licenses you want to buy and calculate the new termination date. You have to be logged in in the dashboard.
https://dashboard.meraki.com/manage/dashboard/license_calculator