Can Android devices connect to both WiFi and mobile data networks at the same time? Can Android devices connect to both WiFi LAN (not necessarily connected to the Internet) and mobile data networks at the same time? Or when it connects to one, does it have to disconnect from the other?
The device is Kyocera Hydro Icon with Android 4.4.2 if it's relevant.
Multipath TCP is a relatively new technology that allows a device to use multiple network connections for the same logical TCP connection. This is used for mobile phones to use WiFi and mobile data connections at the same time to reduce the time for retrieving data. 

I'm not sure which Android phones currently support multipath TCP. 

Without multipath TCP, a TCP connection must be either on WiFi or on the mobile data connection, but not both. It is up to the OS to decide how allocate these connections. I think the standard Android decision is to use WiFi if available (and perhaps with sufficient signal strength) to minimize cost under metered data plans.