1. Actually, it really depends on why and how they have set up a second "modem".
If it is not really being used as a modem at all, and is just being used as a Wi-Fi extender, and all your devices are receiving IP Addresses in the same range (i.e. 192.168.1.xxx), then all device will be seen as on the same network and they will be able to talk to each other. So yes.
If they have actually installed a second modem with a connection to their external network (why they would do that I have no idea), then you would have two networks in your house, with two network IP Address ranges (i.e. 192.168.0.xxx and 192.168.1.xxx), and they would not talk to each other, except via the internet. That would be bad. Possible, but bad.
A third alternative is that they have run Ethernet to your remote listening area and plugged a second "modem" in to that, then set it up to supply IP Addresses to devices in that area. That would be the incorrect way to do it. The DHCP server in the second modem should be turned off, and the first modem should be allowed to provide IP Addresses to all devices everywhere. The second modem would then act as a Network Switch and Wi-Fi Access Point. Then all devices could see each other.
So, you could do some checking of IP Address, and/or you could talk to someone who knows what they have done, and what should have been done.