Detail of connection for a Miyajima mono Premium BE cartridge:
The left hot and ground channels (white and blue connections) are shorted by connecting them together with a piece of heat shrink. Only the right hot and ground cartridge pins (red and green) are connected to the stereo tonearm cable.
This will completely eliminate hum, but we now have only one channel in the circuit! So if you connect your cables as if playing stereo, you’ll get music only on the right side.
What needs to be done is to somehow connect the right channel exiting your phono stage (remember, there's nothing coming out of the left channel) into the right AND left channels of your line stage. That way, both channels of the line stage will receive the sound coming from the right channel of the cartridge.
Most phono stages don’t have more than one output per channel, so you can look for a Y cable that will distribute information from one channel into two channels, but those types of cable are rarely made for high-fi use, so unfortunately there will be some degradation of sound.
A better solution is offered by a phono stage that will solve that problem: an example of such a unit is the Thöress Enhancer. If you take a look at the photo of the back of this unit (click on the link to go to the website), you’ll notice that there are three sets of output, all connected together (“AUSGANG,” on the photo). You then simply use a stereo cable to connect one of the three right channel outputs of the Enhancer into the the right channel of your line stage, and another right channel output into the left channel of your line stage. Now you have beautiful mono sound on both speakers: real dual mono signal.
Note that, when using such a configuration, it is best to dedicate a cable for this mono connection between phono stage and line stage. That way, you don't need to move the interconnects when you switch from your mono to your stereo cartridge. If your line stage has more than one input, simply connect a cable from the second and third "Ausgang" of the phono stage (using both right channels, as described above) into one input of your line stage dedicated for mono duties; and a different cable from the first "Ausgang" of the phono stage (using right and left channel, this time) for stereo duties, into another input of your line stage.
On the other hand, if you use only one cable from the phono stage to the line stage and you forget to re-route the cable when you play your stereo cartridge, you'll get only the right channel information going into both speakers!