$ yes |pv > /dev/null
    46.6GiB 0:00:05 [9.33GiB/s]

    $ taskset 1 yes |taskset 1 pv > /dev/null
    32.9GiB 0:00:05 [6.58GiB/s]

    $ taskset 1 yes |taskset 2 pv > /dev/null
    45.7GiB 0:00:05 [9.13GiB/s]

    $ taskset 1 yes |taskset 4 pv > /dev/null
    45.7GiB 0:00:05 [9.18GiB/s]

Very rough numbers - the 9.13/9.33 difference flip-flopped when I ran the commands again. Binding both processes to the same core is definitely a performance hit though. There might be some gain through a shared cache, but it's lost more through lack of parallelism.

I tried 2/4 as not sure how 'real' cores vs 'hyperthread' cores are numbered. These numbers are from a i7-7700k.