Absolutely use both.
Last week I thought I noticed a crack on one of my HR strap's chest pads and I was ready to order a new strap that day even though I'm on a self imposed spending freeze due to the COVID-19 economic uncertainty. Turns out it was nothing and the old strap is fine but I would feel lost without it since I keep an eye on both HR & 3 sec. power (among others) when I ride and review my rides afterwards.