Unfortunately, without access to Adobe's Device Graph (which is available through Adobe Analytics Ultimate package / Customer Journey Analytics) where you can use a new metric called "people"... there is no way that I am aware of to stitch the different UV metrics for your different platforms together to look for customers that have logged in to both places....
You could try using Report Builder to pull the data into Excel, then use Excel to perform the logic to only show you your customers that have been in both platforms.....