Hey there!
A simple way to do this is use the script event to trigger multiple actions by setting up the condition within the dialog child.
// create
nextroom2 = 0;
//step
if (nextroom2 == 1)
{room_goto(room_2); image_index = new_index; nextroom2 = 0}
//User Event 0 (your dialog script)
[ChooseVariable,object_name,"nextroom2","1"]