The dual 978/1090 antenna is not currently being manufactured by FlightAware. We continue to sell the 1090 MHz antenna via our usual outlets on Amazon and eBay. If/when we reintroduce a dual band antenna it will likely be a different model to address manufacturing issues with the old design.
More support for dual 978/1090 is on our long-term roadmap for the future in anticipation of 2020 in the USA, however I do not have anything specific to announce for now.