Some ideas on 2 people tieups you may be willing to try if you want to. Take your pick on which/how many to do.
A - Place two people back to back. Tie one person's wrists behind their back, in front of the other person's belly (like the other person's body was a pole) and vice versa. Then tie around both people's legs together and to each other, same to thighs, torso, etc. You can do it standing, lying down or sitting (though in that case their legs aren't tied to the other person's).
B - Same as above, only front-to-front (one person's wrists are tied behind the other person's back). Cling film is good here but if you don't have enough use scarves/ties as usual. Then lay down and the tickling can commence.
C - Similarly, have them lay down on their side back to back but in opposite directions. Tie their hands as in A (now they'll be in front of the other person's legs or so), then her ankles/thighs/torso as usual. If you're willing, a final touch: tie each person's cleave gag (with stuffing, of course) to the other's bound ankles

(you need to position them accordingly beforehand). I'd stuff a bandana on each girl and use cling film to wrap around their mouths and ankles together, first in a cleave gag then over the mouth several times (like someone said, secure with a strip of tape if needed).
D - Differently, have them sit front-to-front and feet-soles-to-feet-soles. Tie their 4 big toes together with whatever you can find, like string - or even better, all of their toes together! Then tie their hands in front and connect them towards each other as if they're doing a stretch or trying to hold their hands together - now you can tickle the armpits... Tape their hands so they don't pick at the knots

Finally, tie their ankles and thighs for good measure.
Gag suggestions:
- The one posted in C can be done with the others, just not to the ankles for obvious reasons
- Two socks covered by tape
- One bandana/sock stuffed, covered by tights wrapped many tights around the mouth in a cleave gag
- One sock inside a longer one, then use it as a cleave gag
Hope you like
