Typically, you'd use some kind of stereo effects, and run the SEND from the first amp to the effect, then route the LEFT and RIGHT EFFECT OUTS to the EFFECT RETURNS of both amps.
But wait, there's more! (from the manual):
"DIRECT:
This feature provides a variable strength signal output originating right from the output transformer. Thus, good tone is supplied, all Effects and Reverb are included, and there is absolutely no loss of the Boogie's tone when running from the Direct Output to a mixing board or another amplifier. But bear in mind that speakers themselves exert a tremendous coloration to the tone, primarily by rolling off the highs very steeply above 5 to 6 kilohertz. Consequently, many players will still prefer the recorded sound of a microphone "listening" to the speaker.
In some sophisticated set-ups, players run their Boogie's Direct Out into an effects rack, and then from the last effects device into other external amplifiers. But such a set-up cannot route the last effect output back into the original Boogie. Also note that a speaker or load resistor must remain plugged into a speaker jack at all times, even when using the Direct. Otherwise the amplifier will operate improperly and damage may result from running without a load. Suggested value of the load resistor is 8 ohms, 50 watts minimum.
EFFECTS SEND & RETURN:
These jacks provide a low-noise patch loop within the 50's preamp for
connecting external accessories. Compatibility is quite good with most line-level or rack-mount devices, although some will require you to trim down their input attenuators to prevent overload.
(Pedal-type effects are designed to handle the low-level signals emanating directly from your guitar; therefore this type of device should be connected “in front” of the amplifier, rather than in the Effects Loop.)
(This should be taken with several grains of salt. Feel free to experiment)
Some manufacturers offer rack-mount multi-effects units for guitar which include compression, distortion, overdrive, etc. Although a good-quality compressor can be used effectively in your Boogie's Effects Loop, we do not recommend the use of multi-effect "guitar processors" in the Loop, as their inputs are often too sensitive and their added gain stages will tend to contribute some noise. If you wish to use this type of device with your Boogie, it should be patched in between the guitar's output and the amplifier's input.
(More salt. Once again, it's OK to experiment)
The Send jack can also provide an external “preamp, out” signal without breaking the signal path that leads to the .50's power section and speaker. If you're using the Send as an output but you still want to hear the internal speaker, turn up the Master. If you don't want to hear the .50's power amp and speaker, turn the Master to zero. (Remember to leave the speaker - or a load resistor - connected to one of the speaker jacks).
Plugging a signal from another source into the Return jack provides direct access to the .50's power amp section. Remember to adjust the Master to an appropriate setting for best signal-to-noise ratio."
Hope this helps