A load balancing router would do it... I use one here to aggregate my two crap broadband links into one slightly less crap one!
If you want one PC to connect to both without a load balancer, then you could do it with a multihomed machine (i.e. one with two network adaptors). You could then use the "route" command to add persistent routes to direct certain traffic destinations to one LAN and thence WAN or the other.
(you could could also possibly do it using just one interface if you statically allocate addresses to it, and give it a second IP address that matches the second subnet).