You can try to connect more speakers and blow up the amp, but I don't think I would- Reverb.com has one for sale and the asking price is $499.
If nobody has any emotional attachment to it, I would sell that and look for something that can connect to more speakers, safely.
How loudly does this need to play? If it's ONLY for background, you could find some kind of integrated amplifier or multi-channel power amplifier and add a small mixer that would accept the signal from a few sources, like a Bluetooth receiver, other streaming device, etc.
BTW- the speaker terminals on that Heathkit are marked R/C, L/C and that stands for Right+/Right Common (-), Left+/Left Common (-), so connecting the two wires from one speaker to both Common terminals will result in absolutely no sound coming from that speaker. If you connect the wires from one speaker to the R and L terminals, it will sound like it has an echo and the volume will be weaker than the other two. While it probably won't hurt the amplifier, it might, so I wouldn't recommend doing that. I also wouldn't recommend connecting more than one speaker to each channel, just because A) the impedance (resistance) of the speakers hasn't been measured and that would be needed to determine how safe it would be for the amplifier. Since that amp is about 40 years old, I wouldn't push it too hard. OTOH, the Reverb.com listing has specs and that shows 16/8/4 Ohms as acceptable loads which means you can connect two speakers to each pair of terminals as long as they're each 8 Ohms.
Plug and Play, circa 1973.In its simplicity and functionality, we detect notes of Bauhaus and Braun, and its clean lines place it at home in nearly any setting. Create the perfect minimalist system with this integrated amplifier, a pair of efficient speakers, and a turntable, or a Sonos Connect....
reverb.com