You can't use a 4511 for multiplexing. (well not easily)
You connect the cathodes in parallel. That is display 1 segment a to display 2 segment a, Display 1 segment b to display 2 segment b .... and so on.
Connect these cathodes to a current sink like the ULN2803.
Connect the two anodes to a separate controllable current source like the P channel FETs I had in my monome.
1) To drive it put the anode of display one high (by driving the gate of the FET low) and turn on the current sinks of the segments you want to be lit by putting their inputs high.
2) After a short time say 20mS turn off display one, turn on display two and set the current sinks for the segments you want lit on display two.
After a short time (20mS) turn this off and repeat step 1.
When written in words it looks more complex than it really is.
This assumes you have a common anode 7 segment display, for a common cathode, connect the cathode to the current sinks and the anodes to current sources. As there is only going to be one segment on at a time you can connect the current source through a resistor straight to the arduino output pins.