If I'm interpreting your question correctly, yes. Two concurrent activities can run sequentially or alternately on a single core, via context switching. Or they can run in parallel on separate cores. It shouldn't matter; either way it's concurrency with most of the associated complexity around locks and most kinds of data races. OTOH, the two cases can look very different e.g. when it comes to cache coherency, memory ordering, and barriers. I've seen a lot of bugs that remained latent on single-core systems or when related concurrent tasks "just happened" to run on the same core, but then bit hard when those tasks started jumping from core to core. This stuff's never going to be easy.
