The following video and tutorial were provided by Budget101 Fan Les McMahon who designed and built this bike for a disabled friend.
The easiest method of making this trike is by welding but it can be made just as effectively using bolted construction.
Set 2 similarly sized bikes next to each other 48 inches apart and join together using a 1.5 inch angle frame just behind the headstock.
Position a second frame just in front of the rear sprocket. Make the bottom of these frames level with the ground and 8 inches off the ground.
Fit a piece of timber into the bottom to complete the platform area. Make sure that this platform allows clearance for the pedals to operate.
Remove the front forks, handlebars, crank assembly and chain from the bike that will be closest to the pavement depending upon which country you are in.
The ramp is optional but makes life easier. Using 4 ratchet straps secure the wheelchair onto the platform and put some bags of sand into the seat equivalent to the users weight.
Give each corner a good shake and ensure that everything is tight. Now take it for a test ride and make sure that you give it a good shakedown before taking anyone for a ride.
Obviously I can't take responsibly for how you construct this trike so please be diligent whilst building and obviously very careful when you have your passenger on board.
Have fun and take care !