You could always incorporate a third stringer, similar to a ships
ladder, or even a wood shelf support in the center using the same type
of wood.

If you must glue a few pieces together to make your treads the biscuit
idea would work the best I picked up biscuit joiner at Lowes for about
$100.00 versus the dewalt for $190.00 is not a bad deal if you only
use it a couple of times a year.

Dave FL