Can I wire two Casil CA1240 batteries in parallel?

Yes, you can! And here's how:

With jumper wire, connect the positive terminals on Battery 1 and Battery 2. Connect the negative terminals on Battery 1 and 2. Finally, connect the positive and negative wires to the correct terminals on your device (making sure to observe polarity).

Wiring 6 volt or 12 volt sealed lead-acid batteries in parallel...
will keep the voltage the same but double the Ah output. For instance, wiring two 12V, 4Ah Casil CA1240 batteries in parallel will result in a 12 volt, 8ah (4ah + 4ah) output. Be sure you're wiring batteries that are of the same voltage, capacity, and type. Neglecting to do so will ultimately hurt the performance of your batteries.