Using one pair two sets of wires for each channel

Dear Sir,

I just finnished composing my bedroom system with:

Audio Refinement Complete Int.Amp.
Usher CD100
Kimber PBJ
Wharfedale Emerald 99
and
Homemade Speaker cable (13gauge - not terminated)

The amplifier only has one pair of speaker out while the speaker binding posts are bi-wireable (i.e., two pair of binding posts) - With the given configurations, I am trying to bi-wire my system via applying two sets of speaker cables to each channel. As suggested, the amp has only one pair of speaker out so I have to put two sets of speaker cables to one binding post on the amp.

If I am to use two sets of wires for each channel:

a) Is it safe for my amp or speakers (since the wire becomes too thick [13gauge + 13gauge = ... thick!)?
b) Does it improve sonically or degrade it?

Any comment welcome
best,