Methods and apparatus are disclosed for inserting flexible, multi-lumen catheters into blood vessels, and in particular, for inserting flexible, split-tip catheters into blood vessels. The invention accomplishes these objects by temporarily stiffening each catheter lumen and tip independently through use of intra-catheter stiffener elements disposed within the catheter lumens. This provides means for advancing the catheter/stiffeners assembly through a subcutaneous tunnel, and over a plurality of guidewires until a distal tip of the catheter is at a desired position within the vessel. The intra-catheter stiffener elements are sufficiently stiffening to allow advancing the catheter over guidewires, but sufficiently flexible to allow bending and looping of the catheter for proper placement within the vessel.