Newbie here. I have an old Sony DA3ES Receiver driving an equally old pair of B&W DM640s with B&W 303s for side-positioned surrounds. The receiver has connections for only two 'front' or 'main' speakers so the 303s are hooked up as surrounds. Is there a way to get the signal going to the mains to also travel to the 303s?
Not safely. The 640's specs show that they are a 4 ohm load, while the 303's are a 8 ohm load. Doing the math shows that your Sony receiver would 'see' a combined 2.7 ohm load, which it is probably not strong enough to drive.
In fact it would take one amp with a VERY stiff power supply to drive both those speakers at the same time, an amp that is stable down below 2 ohms.