Firat, you'll need to pay careful attention to the suspension system since it's not designed for a second arm. I couldn't easily find the total weight of a Rega RB301 tonearm but it might be in the range of 500 to 1000 grams. You'll have to add another 500g to 1000g on the chassis to balance out the weight of the tonearm and could be looking at a total of 1 to 2 kilograms of added weight. That's a lot to ask of the stock suspension springs and you may find that the turntable doesn't perform as well. Searching for the correct aftermarket springs could prove to be a frustrating experiment.
I would contact Michell Engineering to see what they suggest before attempting to add another tonearm. Or perhaps you'll find specific information on the topic from someone who has successfully (or unsuccessfully) added a second tonearm.
I thought about adding another tonearm to my Gyro SE but after reading about the potential difficulties, I decided that if I wanted a turntable with two tonearms I'd sell the Gyro SE and buy a turntable that's designed for two 'arms.
Good luck!
Regards,
Tom