That's entirely impossible under EU law. If extradited to UK to Sweden, he can ONLY be extradited from Sweden with the consent of the UK.
Scenario A: The UK would agree (in which case, he didn't need to be extradited to Sweden, as he was already in UK custody).
Scenario B: The UK would not agree (in which case, he cannot be extradited from Sweden).
Nobody has ever been able to outline any way in which being extradited to Sweden could have increased his risk of being extradited to the US. If the US even wanted him; the UK is well known for rolling over to US extradition requests, and he was in UK custody for an extended period of time. The obvious conclusion is that the US is not especially eager to get their hands on him, since they didn't actually try.
"Any extradition from Sweden to the United States would actually be more difficult. [...] there is no evidence whatsoever that the United Kingdom would not swiftly comply with any extradition request from the United States [...] the best opportunity for the United States for Assange to be extradited is whilst he is in the United Kingdom."
Source: http://www.newstatesman.com/david-allen-green/2012/08/legal-...