Road tyres vs "Street" tyres for charity ride on MTB Tandem
It's not the weight of knobblies that's the issue, but how much effort you have to put into deforming them with every rotation - the rolling resistance. The tyres you've got are certainly going to be slower than a wide semi-slick, like many touring tyres, but they'll be a lot faster than tyres designed for muddy trails. The ones you've got don't actually look too bad for road use. 

If you're going to make a habit of riding this bike on roads for decent distances, a tyre upgrade might be in order - you could gain puncture protection as well. As it's a tandem, note the weight recommendations on potential new tyres. 

Overall I suggest you run with what you've got, and if you think "that was good but I'd like to be a bit quicker next time" consider an upgrade. 

BTW Presta tubes don't inherently allow higher pressure, though they're more common on rims rated to higher pressure. Adapting is easy with a grommet but of no benefit to you.