If it is what I am thinking of, yes - it is made to piggyback on an existing
slot. We've used them at work when installing 2-way radios. The fuse comes
out and is put in one slot of the adapter, while the new circuit is fused in
the second slot of the adapter. The fuses lay sideways, on top of adjacent
fuses. The adapter has to go the right direction so the power is picked up,
and that can limit which slot you can use (you can't use the end one or
two). It also blocks access to a couple fuses until it is moved.

Don't use little metal pieces that slip alongside existing fuses - they
irreparably stretch out the socket.

Mike