Use a "Key" to add Total To Another Event?

I have two registrations created

In registration #1 people register their team, but can buy extra tickets to event #1 with a custom field - eb_BanquetTicketsRegExtras .

In registration #2, people only buy the extra tickets.


Is there any way to use the custom field key - act as a counter toward the other event.

Example

Find the file with coding for the registration #2 event capacity and add a math equation.

"Other event maxium capacity" (minus) "eb_BanquetTicketsRegExtras" = "available remaining extra tickets"

Thanks
Tim