parallel payment not working

Dear Support Team,

I have a question about 'parallel payment'. When I do a test with buying two products in two different shops (one local and one global product) and checkout, I get an error.

When I buy the products seperately it works well.

All products are being paid to the same PayPal merchant ID.

Can you help me with this?

Thanks (again...),
Pieter

I've asked this question at PayPal as well, but they are asking "... the API request and response. It's a string of variables and values your store posts to PayPal and the response you receive. The response would include an error message and a correlation ID that I can use to research this further..."

Don't know what they mean.