I am testing the Lync 2013 Mobile client for Android 5.0.8563.0.

I can call two different Lync users using "Lync Call" (SIP call - not EV) and then I can toggle between these two calls and the calls with switch from "On Hold" to "Active".

Is there any way I can connect these ongoing two calls together to a conference call?

I there another way than starting two calls first to set up a conference call from the Android client?

The "Mobile Client Comparison Tables" at
http://technet.microsoft.com/en-us/library/hh691004.aspx
are not detailed enough to tell me if this should be possible or not.

Not possible at this time.

The process to join two calls would be similar to participating in multiparty IM. If you look at the notes it is not possible to initiate from the mobile device: "1 Mobile users cannot initiate multiparty IM or add users to an existing IM, but they can participate in multiparty IM if invited."