International Plan for 2 and proration questions

We have several phone lines on our account.  2 of the phones will be used overseas for 10 days only (Taiwan and Philippines).  We currently have a plan that shows that data and unlimited text are included.  And it shows 25 cents per minute otherwise.  

Does both phone lines need to sign up for the $35 plan if we want to use the phone like we are in the U.S.?   Meaning $35 x 2?  Is there a "family plan" for the trip?  Is the rate pro-rated if we are only gone part of the month and cancel as soon as we get back to the U.S.?  

If we decide not to sign up for the international plan, can we turn off incoming and outgoing calls and just use messenger or viber to connect so that we are using purely data only?  We only need this to check kids for back in the U.S., making local calls in both Taiwan and Philippines and for using directions/map applications such as google maps.  ¡Gracias!