I remember seeing this ring built specifically for this purpose. It was an unobtrusive metal band and you could synch it up to its pair and it would squeeze your finger in time with the pair's heartbeat sensor, I'll see if I can find it again
I don't believe this is the one I came across but was all I could find with a quick search. The website seems a li'l sketchy based on grammatical errors so I'd do some double checking into it but it seems like what you're looking for. https://my.thetouchx.com/hbring/howto-use-hbring