> DHCP assigned them the same IP, and google just continued the session which had been left open

Given that you're using DHCP-assigned IP addresses, you're likely behind NAT. From Google's vantage point, both employees will appear to always have the same IP address as each other.

> is it possible that a session collision happened

Looking at my browser's session state, I suspect the session cookie is SSID, which is 21 bytes. Assuming it's a base64-encoded integer, it would be a 14-byte integer. I think that would make a collision a fairly rare occurrence.