I've already answered on how to clone AP (merge with the original on a wireless network list) in here I think.
If you're evil twin is an Open Network (waiting for the user to mistakenly selecting it manually from the network list), you would have to keep sending deauth frames until the reconnection time times-out.
If you want to automatically accept the request of authentication (which will be passed to the RADIUS-AAA server) from the client you need to have exactly the same settings on the AP.
- Higher signal strength (There is a parameter that says which AP is better to connect "roaming aggressiveness")
- Same ESSID (The name of the network the probe requests look for or the beacon the AP emits)
- Encryption algorithm (WPA2, judging by the tag on your question)
- The Cipher (CCMP)
Which roughly translates to this on the hostapd.conf (These are SOME parameters)
auth_server_addr=192.168.0.1 // RADIUS SERVER (Enterprise / 802.1x)
auth_server_port=1234 // RADIUS SERVER (Enterprise / 802.1x)
auth_server_shared_secret=verySecret // RADIUS SERVER (Enterprise / 802.1x)
wpa_key_mgmt=WPA-EAP // RADIUS SERVER (Enterprise / 802.1x)
ieee8021x=1 // RADIUS SERVER (Enterprise / 802.1x)
wpa=2 // Encryption Algorithm (WPA2)
wpa_pairwise=CCMP // Cipher (CCMP)
ssid=evilTwin // ESSID
Here's the link where I took this example from (for further knowledge too).
Now, this obviously will fail at some point because you need the RADIUS server, you're talking about 802.1x, so you also need to configure the
eapol-config file (If you're using freeRADIUS) which changes everything when you choose one of the 22 supported EAP methods (EAP-PEAP, EAP-TTLS, EAP-TLS, EAP-MD5, EAP-FAST, etc.)
Because the PSK generated from the fake AP would be different from the one generated by the client.
The key used for data encryption is called PMK, and it's dervived from the TLS Masster Secret in EAP-TLS, here you can find some more infor specifically for EAP-TLS
I hope this answers your question because it's still missing what you want to achieve and everything about the RADIUS server.