Setup example: two "unisex single+couple" prims (Single Avatar Multisit script):

PRIM 1 NOTECARD:

/f
-2
+Single
<0.07842, -0.96654, 0.75317>,<-0.00000, 0.00000, -0.00000>,singlem,1320,Single M
<0.07842, -0.79174, 0.79224>,<-0.00000, 0.00000, -0.00000>,singlef,1284,Single F
+Couple M
<0.23632, -0.62757, 0.99316>,<-0.00000, 0.00000, -0.00000>,couplem1,1316,Couple M1
<0.07842, -0.37320, 0.94434>,<-0.00000, 0.00000, -0.00000>,couplem2,1356,Couple M2
+Couple F
<0.07842, -0.63820, 1.01123>,<-0.00000, 0.00000, -0.00000>,couplef1,1274,Couple F1
<0.07842, -0.73724, 0.84424>,<-0.00000, 0.00000, -0.00000>,couplef2,1385,Couple F2

PRIM 2 NOTECARD:

/f
-2
+Single
<0.07842, -0.96654, 0.75317>,<-0.00000, 0.00000, -0.00000>,singlem,1320,Single M
<0.07842, -0.79174, 0.79224>,<-0.00000, 0.00000, -0.00000>,singlef,1284,Single F
+Couple F
<1.07842, -0.63820, 1.01123>,<-0.00000, 0.00000, -0.00000>,couplef1,1274,Couple F1
<1.07842, -0.73724, 0.84424>,<-0.00000, 0.00000, -0.00000>,couplef2,1385,Couple F2
+Couple M
<1.23632, -0.62757, 0.99316>,<-0.00000, 0.00000, -0.00000>,couplem1,1316,Couple M1
<1.07842, -0.37320, 0.94434>,<-0.00000, 0.00000, -0.00000>,couplem2,1356,Couple M2

Note: the 2 single poses are a male and a female sit, and the proper one will be remembered! Each time an avatar stands up while using a single pose, it is memorized and used as default on sit, for the 100 most frequent users! This setup also allows swap of couple poses while sitting!
Tips:
You can easily "force sync" a "single" avatar when the other chooses a couple pose: see the scripting examples page.
If the object has only these two sit prims (or if you can link them as root and 1st linked prim...) you may prefer to use 2 prims with the same position and rotation to save time: you will need to set up the couple poses only once (just paste them on both notecards, swapping male and female on the second prim).
If the two prims are identical "cushions" placed on a "sofa" instead, you can set up the single poses only once...
Remember: adding the "root touch" script to the root prim will make all prims touchable to get menu...
Remember: the negative number (and the number of poses) must be the same on "single+couple" prims.

Same as above, but with single pose choosing from main menu ("same name" trick):

PRIM 1 NOTECARD:

/f
-2
+Single M
<0.07842, -0.96654, 0.75317>,<-0.00000, 0.00000, -0.00000>,singlem,1320,Single M
+Single F
<0.07842, -0.79174, 0.79224>,<-0.00000, 0.00000, -0.00000>,singlef,1284,Single F
+Couple M
<0.23632, -0.62757, 0.99316>,<-0.00000, 0.00000, -0.00000>,couplem1,1316,Couple M1
<0.07842, -0.37320, 0.94434>,<-0.00000, 0.00000, -0.00000>,couplem2,1356,Couple M2
+Couple F
<0.07842, -0.63820, 1.01123>,<-0.00000, 0.00000, -0.00000>,couplef1,1274,Couple F1
<0.07842, -0.73724, 0.84424>,<-0.00000, 0.00000, -0.00000>,couplef2,1385,Couple F2

PRIM 2 NOTECARD:

/f
-2
+Single M
<0.07842, -0.96654, 0.75317>,<-0.00000, 0.00000, -0.00000>,singlem,1320,Single M
+Single F
<0.07842, -0.79174, 0.79224>,<-0.00000, 0.00000, -0.00000>,singlef,1284,Single F
+Couple F
<1.07842, -0.63820, 1.01123>,<-0.00000, 0.00000, -0.00000>,couplef1,1274,Couple F1
<1.07842, -0.73724, 0.84424>,<-0.00000, 0.00000, -0.00000>,couplef2,1385,Couple F2
+Couple M
<1.23632, -0.62757, 0.99316>,<-0.00000, 0.00000, -0.00000>,couplem1,1316,Couple M1
<1.07842, -0.37320, 0.94434>,<-0.00000, 0.00000, -0.00000>,couplem2,1356,Couple M2