Our hallway has one small ceiling light that is controlled by either of two wall switches at opposite ends of the hallway. I want to add an additional light fixture toward the other end of the hall & have both switches control both lights. Thanks.
There will be a pair of conductors coming into the light box, one of which is the switched hot. You can tie into those to add another light either in the light box or in the switch box at the other end of that cable.
Be sure you have an understanding of box-fill regulations, connection techniques, etc. before you make any changes. For further assistance, show a diagram of the current situation.
If you have access to the attic above you can just add a fixture box where you want and wire it to the existing lights wires. The switches will now power both lights.
If you don't/can't get into the ceiling they make external conduit systems like the one below, but on the ceiling with light fixture boxes instead if outlets.