I also agree the phone should behave as a single SIM if the other one is not being used. I think they did that so people would know you can use one more SIM - yeah, most people think they know how to use phones, but they don't. Or they simply forgot about offering a 'hide' feature.
BUT you can open one of the Phone and Messaging apps, touch ... and then 'Link tiles', so you will not see a number 1 floating near both icons. Hope it helps.