I got a few interesting informations, regarding the Gc-100 devices which only allow 1 connection at the same time. There is a setting for the device (open up the extended device menu with the small square / arrow icon) to 'disconnect after each command' - this should allow you to use 2 iphone or ipad apps at the same time. It also works with av receivers which also only allow 1 connection at the same time.
Furthermore Andy told me what to do, in order to use remote access (when you are not at home) :
In the same menu for the device, you can set up the remote location information - put in the IP address / port for each device (the IP address will always be your dyndns host name), and make sure you port forward all the ports on you router to let the data through. Then, whenever you want to control the system remotely, go in to the settings for the app & turn 'remote access' ON - and the app will start using the remote location IP address / port instead of the 'normal' IP address / port.