How do I get my forces and ships back after I support another city?
From
City View, click on the
Agora
Click on the
Outside tab
you should see a list of every city you have tripwires or support troops in, and a rundown of what troops/ships are there .. find the one you want to pull the support from and click on one of the two arrows to the right .. the large arrow on top is return all support, the smaller arrow below it to return some of the support (it lets you chose) .. also if you plan to leave a tripwire in place do not select all
It will ask Do you want to withdraw units .. select yes or no
How can two cities combine there forces to attack another or support another city?
You can send support from more than one city, and multiple players can support the same city .. but if you mean is there a single command to combine them all, the answer is no. If you meant something else please explain further.