WICED SDK v2.4.0 (I also looked in v2.4.1 and its still there) in file: WICED\Wiced-SDK-2.4.0\Wiced-SDK\Wiced\Network\NetX_Duo\wiced\tcpip.c, function wiced_tcp_listen(), line 303. Interface is hard-coded to WICED_STA_INTERFACE.
The setup: bring up both the STA and AP network. Start an STA http server. Now attempt to start an AP http server. This will fail with a WICED_ERROR returned from wiced_tcp_listen() because the STA socket has already been initialised, and due to the hard-coded interface, that's the only socket checked by wiced_tcp_listen(). I haven't tried it, but I suspect that you could also erroneously create two or more AP http servers because of this bug.
Effect: Using NetX Duo, I can start both an AP and STA http server only if I start the AP server first, the other way around the AP http server does not start.