Methods of forming catheters are disclosed, together with methods of forming split tip catheters. In one aspect of the invention, the manufacturing methods can include the steps of: providing first and second catheter tubes each having a substantially D-shaped cross-section, and attaching at least a portion of longitudinal lengths of the first and second catheter tubes along flat surfaces of the first and second catheter tubes to form a dual lumen catheter assembly. The tubes can be fused along at least about 10%, preferably along at least about 50%, more preferably in some applications along at least about 70%, 80% or 90% of the longitudinal length.
