If I understand your current setup correctly (HDMI signal going from the X1 box in to the Xbox One, and from the Xbox One to the TV), then I think we should stick with the theme and go with one HDMI converter. The reason being that the HDMI converter (at least the one we recommend) doesn't degrade the signal, and I assume the Xbox doesn't either. The Xbox should fit seamlessly into the setup.
First, I'll just mention this article that discusses the HDMI converter that we recommend for use with the Slingbox:
Basically, the converter will fit into your current X1 --> Xbox --> TV hookup by inserting the converter between the X1 and the Xbox. The converter will also have an additional output you'll be able to hook the Slingbox into.
Hope this helps!