You are talking about a whole different kind of error chip.
2 different denominations on a NV chip is much more valuable than a double sided chip. Double sided chips are much more common than 2 denominatios. In most cases there can not be only one, double sided chip. There has to be 2.

The Fiesta Charter Member chip was LTD 200. There were 6 double sided chips, 3 of each side.

I am interested to know what you base this claim on:
"I have what I know is a one of kind Horseshoe Tunica error chip ($2.50 one side, $5.00 reverse)."
1 of a kind is strong words.
Also have you ever recieved a offer of $300 for it? I have seen 2 denomination Caesars chips ($5 & $25) sell for a lot less than that.

I have a CA 2 denomination chip that I thought was one of a kind for years. In the last 2 years I have seen 8 of them.

The problem with error chips (except those nuts in AC <g>) is, everybody wants them, but they will not pay to get them.