How can I prevent incoming calls to two phones?
You can't prevent incoming calls from reaching the phone, at the phone; that would have to be done at the switching office.  You might build something to block it at the phone, but it would be much simpler to incapacitate the ringer.  In either case, you'd be unable to an outgoing call (on the same line) while the incoming call was being blocked or trying to ring.  The only difference: a blocker might be able to isolate the phone so that picking it up to make an outgoing call while in incoming call was being blocked would get silence.  Merely stopping or removing the ringer would still leave the possibility of you inadvertently answering an incoming call.