Concurrency within Groovy Script

I have a question regarding concurrency within my Groovy Script. I am beginning two threads, which both run at the same time, and run until they have finished their tasks. I begin these tasks as following :

send.start()
receive.start()

Both of these tasks run independently - although they do send separate SOAP requests to the same external server IP address simultaneously from my machine.

I do not receive errors from Groovy when these tasks are run. However, within a few cycles of each process, the 'SOAP' icon of one of the requests utilised by the processes turns red with <mising response data> in the reply. The request which this happens to is seemingly random each time, and the other request task will continue successfully until finished.On checking Wireshark at both ends, the packets for both requests are definitely being delivered as expected, and server is replying as expected. The error is definitely within SoapUI (or rather - my implementation within SoapUI).

Both the send task and the receive task work 100% of the time every time when run individually. Just when they are run together in the way shown above the requests seem to corrupt each other.

Any advise or input would be great.