In order for you to see any performance increase by plugging in two cables between the switches, you need to set up something called link aggregation, which will increase throughput between the two switches, and also provide redundancy if one of the links fail.
Link aggregation is not supported by unmanaged switches, you need a managed or websmart switch.
Here is an example on how to set up link aggregation between two switches.
As noted by davidgo in the comments, plugging in two cables between switches without using link aggregation is actually a terrible idea.
Smarter switches (ie, running STP - spanning tree protocol) will hopefully realise whats going on and disable one of the two links. Dumber switches may get confused and start sending the traffic in a loop between the two switches, which is a great way to make your network turn into a notwork!