2 LAN subnet : enable communicating together

We have 2 DSL line with 2 different providers. the first DSL line is used for internet access for all the PCs, and its LAN for printer and file servers. The second line is used for VOIP and eventually backup of the other line if it fails.

Right now each DSL line has a its own router and separate LAN subnet (Subnet 1 : 192.168.0.X; subnet 2 : 192.168.1.X).

The problem that we have now, is that when the main DSL line fails we manually swich the office LAN to subnet 2, so we have to change the configuration on the PC .. and we can’t access servers and printers that are still configured on the subnet 1.

Would it be possible to connect together the 2 networks and access IPs from all subnets?
Or do we need to buy some equipment ?