When extending a wrench using a breaker bar, should I connect the two with a spline bit?
The open end on a wrench is created to take only so much force before the jaws start to spread. If you need a Swede bar on the wrench, dispense with using the wrench.

Using a Swede bar on open end wrenches guarantees enough spread that they start rounding bolts after they've been permanently deformed. Wrenches are sized (jaw thickness and length) so the maximum torque using normal force will be about the range the bolt should be tightened to if you were to consult a bolt torque chart.

For similar reasons, you probably shouldn't be using Swede bars on box end wrenches.

For difficult to loosen nuts and bolts, you need a socket on the breaker bar and the socket directly on the bolt, preferably a 6 point socket so you don't round the hex. 

Breaker bars are used because they're longer (more torque) and have no ratchet mechanism to break. You use them with a socket because the socket fully surrounds the nut and won't splay under the force (though I've split both 6 and 12 point sockets). 

The 6 point socket usually is physically stronger than a 12 point and has larger engagement area (less likely to round the hex), though the newest technology is spline sockets which are designed to engage both nuts and bolts 1/3 the length of the flat back from the point and potentially are less likely to round off hex fasteners.

If you must use an extender (Swede bar), use it on the breaker bar as it is the most likely device you have that will take the excess torque. If the breaker bar breaks (I've done that as well) you need to graduate up a size on the drive (3/8" to 1/2") and try again. 3/8" drive stuff is for 5/8" and below, use 1/2" drive for 3/4" and above.

Consider using penetrating oil and mild heat if these fasteners are large and refusing to move. Also, how much rust buildup is there?