Can I use a USB to HDMI converter to set up 2 external monitors on my 2015 Macbook Air?
The MBA GPU may be limited to a single external monitor, but the USB to HDMI requires a driver installed on the MBA and that video/image is not using the GPU, it's using the CPU. So the limit is bypassed. Now it might be more complicated than this, but I think is this the basic premise. Be aware that for fast moving videos or games the USB connected display may disappoint. Also if the MBA is running near 100% CPU usage and then you add this USB display it may also disappoint.
If you looked to add more screen for email, YouTube it's likely to be fine.  The hub might be more of an issue, if the cable is USB 3 make sure the hubs too