If I was to build one - I would possibly suggest, putting the headset and forks onto the bottom bike, then welding an old front axle to the top of the headtube. Use the old axle to bolt on the forks of the upper bike.
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If I was to build one - I would possibly suggest, putting the headset and forks onto the bottom bike, then welding an old front axle to the top of the headtube. Use the old axle to bolt on the forks of the upper bike.