How can I use one lvmcache "cache pool LV" for multiple origin LVs?

You can create cached volume and convert it to thin-pool.

cache pool (cache data + cache metadata) -> cached volume (cache pool + original big slow volume) -> cached thin-pool

In this example vg0 is your volume group with physical volumes /dev/small-fast-disk and /dev/big-slow-disk. All logical volumes must be in the same volume group.

create cachepool volume (cache data and cache metadata in one command)

lvcreate --type cache-pool --name cachepool --size 128G vg0 /dev/smal-fast-disk

create cache volume

lvcreate --type cache --cachepool vg0/cachepool --size 3T --name thincachevol vg0 /dev/big-slow-disk

convert cached volume to thin volume

lvconvert --thinpool vg0/thincachevol

On a mainline kernel using the same cache for multiple origin devices is NOT supported by dm-cache (which is what lvmcache uses underneath) due to races accessing metadata. See Re: [dm-devel] dm-cache: can the same cache be used with multiple origin devices? for a 2013 comment from a developer.

However, for completeness the original dm-cache developer has an abandoned testing repo supports "multiple source devices on the same cache device" (see https://groups.google.com/forum/#!topic/dm-cache/q-lM1t438PU for details) but this code is not in mainline kernels.