Can Android devices connect to both WiFi and mobile data networks at the same time?
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.