marketpress checkout from multiple stores using chained payments?

Is it possible to set my website up so that a customer can purchase from multiple stores in one checkout process using marketpress and chained payments?

Currently I can add a product from one store and successfully checkout using the paypal chained gateway - which is a great start. But I want customers to be able to purchase site wide.

I have tried testing this and can't get it to work.

I add a test product from one store and all is fine, add from another store and the first product disappears from the cart - I can checkout the second stores product but the first has disappeared.

I tried following this advice and resetting global cart on network admin. This allows me to add multiple items to cart successfully but stops working at the point of payment and gives me a paypal error.

I need this to work, I would really appreciate any help.

Thank you