No, that won't work that way. Your switch will select one of those two as default, most likely the one with the lowest IP address. One way to solve this is of course buy a real router capable of handling two full feeds, but there are other solutions which may work. You could ask both your transit providers to announce the default route as two /1's or four /2's (etc). That way you can use local preferences to prefer parts of the routing table via each provider. Of course, load balancing based on prefixes is no guarantee that you're actually balacing traffic ratios evenly.