hath wroth:

For an experiment I'm conducting I want to
be able to transmit two data streams from two WiFi(b) routers at the
same time.

Through the same antenna or seperate antennas? If one antenna, use a
Wilkinson combiner to isolate the xmitters so there's no intermod.

I realize that they have carrier sense avoidance so that if
they are right next to each other they will not transmit at the same
time.

Sorta. 802.11 is CSMA/CA or collision avoidance, not collision
detection. It will do it's best to prevent simultaneous
transmissions, but it still can happen. The receiver will listen for
data, but will only wait until the end of the random contention time
period. Allegedly, it will wait 3 contention periods before
transmitting. Unfortunately, I've found that it will transmit anyway
after these 3 periods. This is to prevent broadband noise from
creating the ultimate denial of service attack and shut down all
wireless transmissions by transmitting continuous noise.
http://www.zytrax.com/tech/wireless/802_mac.htm

Is there a piece of hardware (filter, diode? or something?) that
i can put between the output of the routers and their respective
antennas such that this device will pass the transmit signal to the
antenna but not pass any radiation received from the antenna to the
routers?

Others have suggested a circulator or isolator. That will work.
So will a hybrid ring.

The idea then would be that the carrier sense would not come
into play as no power would be received from the antennas.

It might be easier to simply disable the receiver section of your
unspecified wireless device. Many Wi-Fi router use seperate RF power
amplifiers in the TX path. That means there's an exposed T/R swich
somewhere on the board. Find it, chop the RX input line, ground or
terminate the RF input pin, and you're done. If you can identify the
radio chip, you should be able to easily find the RX pin.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558