I laid the two pieces back to back and lined them up very carefully. It is crucial that the tops of the two pieces are level and lined up. As in the picture, I used a strip of UHMW on each end to separate the two aluminum pieces 3/8". This enables me to be able to use 3/8" diameter bolts to hold down the headstock and tailstock to the lathe bed. I used three smaller bolts rather than one large bolt through each piece of UHMW to prevent rotating.
I numbered each intersection of the base in case I ever need to take it apart, it would be easier to put back together.
The first picture is a close-up of one of the two UHMW pieces on the end. The second photo shows the bed being stood up by a clamp.