Can I hook up two amps to the same speaker?

Any audiophile approved speaker cable can be used with the switches.
BUT, you would need to have some 'extra' wire of the same type. (a few feet would do.)
Locate your 'switchboxes' right up close up to the speaker, one behind each speaker.
The dpdt switches would be in a small box from Radioshack.
The side of the box could be quality 5way binding posts WBT would be good. Then a bit of audiophile wire soldered from the 5way to the switches, then use a longer bit hardwired to the switch (Remember, this is from the center terminals of the three pairs on the switch bottom) to terminate with spades or whatever right on your speaker terminals.
So:
You need:
Two small aluminm boxes about 1.5"x2"x3" from Radio Shack.
Two DPDT switches rated 120v are fine, also from Radio Shack. (you could go for some really expensive silver contact dpdt switches, but I do not know where you could find them?)
(expensive parts:) EIGHT pair of binding posts. You can go cheap from Radio Shack, or go for WBT at up to $40 each.
The extra speaker cable you need about 3 to 6 feet of cable (but you want the box on the floor right behind the speaker, so whatever it takes to go up to the binding posts from the floor.) (mine are TWO FEET off the floor)
Any electrical type person can drill the holes in the boxes, wire up the wires to the binding posts, switches and the straight from to output of the switch (the center pair of three pair on each switch THIS IS IMPORTANT) with spades or whatever to connect directly to the speaker posts.
===========================
one set of the receiver A right speaker goes to the right side box posts A. The other receiverB goes to the right side box posts B. the switch in the box uses the receiver output you select and leaves the other circuit OPEN (which is fine if you accidentally leave the other receiver on.
So both are never shorted to each other, but only one at a time is powering the speakers.
You could also use a SPEAKER SELECTOR SWITCH but would have to install it "Backwards" As you want 2 receivers and one speaker pair, instead of one Receiver and TWO speaker pairs.

Elizabeth, are the switchboxes required to use as a safty feature in case both amps are left on? If one amp is off why would their be a problem connecting both spades directly to the binding posts of the speaker?

If you leave both amps connected to the speaker, the one powered up will likely damage the output stage of the one powered off. That's why you need a switch.

Also, if either amp uses tubes in the output stage be sure that the speaker is connected to the amp before powering up or you could damage the output transformer. No such problem with solid state outputs.

Ghostrider, I understand now the reason for the switch. One of my integrateds is tubes. Wouldn't you hook up the spades with power off for safty, anyway? Once the tube integrated is hooked to the swith box, powering up is not a factor, correct? It would seem that this, once the deed is done, to be a good way for me to have tubes and ss. I'm a little concerned that the switchbox is a kink in the signal--amp to speakers. Am I over reacting?

May I add that the switch you seek should have a 'break before make' not the 'make before break' type circuitry.
Here are the make before break/break before make definitions:
In a make before break type, a contact will not open it's primary output circuit until it has closed it's secondary circuit. Conversly, in a break before make type a contact opens the circuit to one output before it closes the circuit to a second output.
Also a DPDT with a "OFF" center position will work.