Wire Two 12v Batteries Together in Parallel With Solar Panel and Raspberry Pi
Your circuitry for two batteries is correct. 

Load: 4W x 24h = 96 Wh.
If this is to be 20% of battery capacity you need 96/0.2 = 480 Wh battery capacity for 24 hr operation.
12V x 18 Ah = 216 Wh
so two = 532 Wh (nominal) so about right.   

If you have rainy/cloudy days with below enough input you will drop under 80% SOC - which is OK occasionally. 

In Windsor With say 4 sunshine hours (equivalent full sun) to get 480 Wh a day in 4 h you need 120 W input. That's non trivial but also very doable.
As a starting rule of thumb until you address specifics use 1.5 to 2 x the PV capacity compared to what is used - so 180- 240 Watt PV panels.
If the charge controller is NOT MPPT you lose about 30% from a 12V nominal panel as these are 18 Vmp.
A say 200 Wmp PV panel is a good starting point. 

Here are solar etc conditions for Loveland - just down the pike :-)
Gaisma loveland

The 6th chart shows sunshine hours/day average by month  



You get over 4 SSH/day March to September. Design for 4 hours if winter no needed. Ask if more input needed.