I don't think it would be allowed as they negotiate as a bloc - but presumably they are allowed between two EU states because UK has the Le Touquet agreement with France about the border posts but that would have nothing to do with any other country anyway.
This report about the Brexit talks etc popped up in my feed to day - could not do a link so had to copy it all - Verhofstadt.
Press Releases OTHERS 5 hours ago
The Brexit Steering group met Michel Barnier, EU negotiator for Brexit on Tuesday 25 July and issued this common statement after the 2nd round of negotiations between the EU and the UK.
“Last week was the first real round of Brexit negotiations between the European Union and the United Kingdom. It was an opportunity for the two parties to review the main issues and map out where further explanations are necessary,” said the Members of the EP Brexit Steering group.
“However, if we want negotiations to succeed within the limited time we have, progress on more detailed content will have to be made sooner rather than later. We can only start talking about a new EU-UK relationship if sufficient progress has been achieved in the three main withdrawal areas: citizens’ rights, the financial settlement and the border issue on the island of Ireland.”
“The European Parliament cannot be clear enough that sufficient progress means progress across the board, and not just in one or two areas. The European Parliament will formally and in due time indicate when the point of ‘sufficient progress’ has been reached”
“To be precise, the European Parliament will remain vigilant regarding citizens’ rights and will continue to push for full rights for EU citizens in the UK as well as UK citizens in the EU. It is a core mission of the European project to protect, not to diminish, the fundamental rights of all citizens."
“The European Parliament specifically seeks to fully safeguard the rights concerning family reunion, comprehensive healthcare, voting rights in local elections, the transferability of (social) rights, and the rules governing permanent residence (including the right to leave the UK without losing this status). Simultaneously, we seek to avoid an administrative burden for citizens and want proposals which are intrusive to people’s privacy off the table, e.g. proposed systematic criminal checks.”
“Last but not least, the European Parliament wants the Withdrawal Agreement to be directly enforceable and to include a mechanism in which the European Court of Justice can play its full role.”
Note to editors
Members of the Brexit Steering group:
Guy Verhofstadt
Elmar Brok
Roberto Gualtieri
Philippe Lamberts
Gabriele Zimmer
Danuta Hübner