Any one using 2 separate amps using a/b switch to same speakers?

Good points. My pre does have two amp outputs, however, the second goes to my sub.  Switching out rca’s And Bananna plugs are really not that big of deal. 

You might try a TC-7220 2-Way Amplifier Speaker Selector Switch Switcher Comparator 

I am pretty far down the Audiophile food chain but,I use it to select between a Primaluna Prologue 5 and Emotiva XPA-100[x2] amps.

Niles Audio SAS-1
To be caution most tube amp and some SS amp will damage without load.

IMO the Niles gear is not all that great - lots of noise and potential for damaging your gear.  FWIW I wouldn't use one for any valuable gear.

You don't mention if you are using a preamp or integrated systems.  If you are using a preamp the ideal way to do this is if your preamp supported more than a single output.

If not and you really want to do this I suggest that you look at the Luxman AS-4III or AS-44 RCA switchers.  It's very good and is not too expensive on eBay from one of the reputable Japanese sellers.

You can also do this a different way, via speaker outputs.  The Luxman AS-5III, AS-50R (remote) and AS-55 are very good products, as are all things Luxman.  I own an AS-50R that I use for comparing speakers and it works great.

You might also look for a used Manley Skipjack RCA switcher.  This was probably the best unit of it's type but it was sort of expensive.  They occasionally come up for sale used but they tend to go quickly.