Using 2 fibre lines with flat VLAN

Hi guys,

Need some advice on a new network setup for a small company I sorry. Currently, they have a fibre line going into a router with a 10.0.0.254 range. This is going into a stack of switches which all the PC's and IP phones go into and the Server which handles DHCP for the PC's and IP phones.

They have just install a 2nd fibre line and want to separate their traffic so VOIP goes down the old fibre line and all other data goes down the new line. Will this be possible using only a flat VLAN setup? My manager is adamant he doesn't want to go down the multi VLAN route. We have a Sophos SG that we can use as a router for the 2nd line. They have 52 IP phones which we can set to with use DHCP or static IP's and about 50ish PC's.

Here is a digram i drew of the setup: http://imgur.com/gallery/W4iJEas

Any help would be greatly appreciated.