Linux access to interfaces

As I understand it only one process can access a network interface at any given time.

I'm not aware of such a limitation, at least not for Linux. You can start as many instances of Wireshark and packet generators as you like and they will all have access to the same interface.

Does this mean running wireshark and a packet generator on the same interface will half productivity?

well, it depends on how you define "productivity". Wireshark is a passive packet sniffer, so it will not consume any bandwidth. It will however consume system resources (cpu, ram). And in that respect, it will reduce the overall "productivity" of the system.

Regards
Kurt