I like a board of around 1400mmx380mm and I'm currently 98kg. Add 2kg for harness etc so you can round it to 100kg.
Area of above board is
1.4 x 0.38 (ignoring reduction in area near tips and ends of sides)
=0.532 sq meters.
Assuming you have a 90kg rider and a 60kg rider,
you'd need about half again as much area, say 0.8 sq meters.
My board length / width ratio: 1.4 / 0.38 close to 3.7
Could buy a closeout/used door and attach points to it for second footstrap set.
I have placed (in a pinch) tandem straps using the existing inserts for the grab handle, to middle pair of strap holes on the board. But -this puts the passenger basically even with your feet, not great...new and more toeside placed attachments are better of course, but if you just want to try it out that does work.
Definitely add ‘extenders’ straps on the bar ends so you can sheet without leaning into the passenger making it awkward. And be ready for insane bar pressure, put comfortable grips on them.
BTW- a Below the bar trim is great for tandem.
I ended up making a split travel version out of an existing crazyfly door and put the passenger footstraps on the scab.
It works great. Should probably post pics of that, Ill get around to it!