Is there a device, something like a dummy load, that will allow one to test two radios back-to-back without going over the air?
If you use a dummy load on the transmitter, it is likely that there will be enough power and enough leakage from either the load or the transmitter itself to be easily received. This is quite likely for the power levels and shielding quality in amateur equipment (homebrew or commercial).
If you had a non-leaky or very low-power transmitter (e.g. the various SDR transceivers recently popular), so that you actually do need to make a connection, then what you need is an attenuator. This is essentially a “pass-through dummy load”: it has two ports and an internal resistance, all properly impedance matched, so you just connect it between the transmitter and the receiver, and the majority of the power goes into the resistance.
An attenuator used for this purpose must have the following specifications:
Enough attenuation (loss) so that the power output of the transmitter is reduced to the safe input level of the receiver, but not so much that the signal is below the noise floor of the receiver.
Like a dummy load, it must be able to dissipate the full power output of the transmitter (the fraction going to the receiver is insignificant).
Attenuators can have either fixed or variable (manually or electrically) attenuation. Variable attenuation is very useful for testing but can be very expensive since they are fairly obscure test gear. I found one for cheap at a flea market that I am informed probably used to be a component of a larger unit, but absent such luck I would suggest purchasing a fixed one of the appropriate value.
That is, if the dummy load doesn't work to start with.