Plug the two networks together, turn off DHCP on one of the routers, give them both static IPs in the subnet of the router which still has DHCP on, but with different gateways.
ie
1GbRouter 192.168.0.1, DHCP active on range 192.168.0.100-200
6MbRouter 192.168.0.2
Your PC 192.168.0.3, gateway 192.168.0.2
Your wife's PC 192.168.0.4, gateway 192.168.0.3
All with a subnet of 255.255.255.0 and, if asked, a broadcast address of 192.168.0.255
Everything is on network, but you can set which connection they use by setting the gateway to the IP address you want. If you plug a gigabit switch into both PCs and one of the routers, you'll even be able to transfer at 1Gbps between PCs for about £12. If you plug anything else in, either set the gateway to choose which connection to use, or leave it as DHCP to use your 1Gbps connection.
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Abstract description of what's happening, to see if a visualisation helps.
Think of it as one house with two exterior doors - a big patio door, and a catflap - plus a load of interior doors. Everyone in the house can walk anywhere else in the house through your interior doors, but to go outside you can choose whether you want to run through the big open patio door very fast, or squeeze slowly through the catflap.
The patio door and catflap are your connections to the internet, the internal doors are your routers/network cables/switches etc.
Now, say you want to get from just next to the patio door, to just next to the catflap. You have two choices - you can either walk through the house (nice and fast, 100Mbps or 1Gbps depending on your network) or you can go out of the patio door (fast, 1Gbps connection) then round the house (internet) and through the catflap (6Mbps) - this means that you're limited to 6Mbps. It's pointless, don't do it.
