Actually, it is a tricky question and best answer would be: it depends.
The first though is - we are agile and communication with customer is the key to success!

But wait a minute, there are some issues to be considered, mainly, who is allowed on customer side to make decision? Is it only one person or there is a team that can be contacted?

In general, the best solution usually is tailored for given customer and/or contract but the usual approach is to establish contact between project managers for planning and heavy decisions when allowing clarifications and technical discussions between team members.