When me and my GF ride balloons together, we usually go with something large like a GL500 or 700 and like someone above said keep a few point of contact with the floor as to keep a lot of weight off the balloon. We usually sit facing each other with our legs behind us in a kneeling fashion, except with huge balloons like GL1200 where my GFs feet barely touch the ground.
For round balloons, it definitely depends on you and your partners weight and body types, I’m tall and about 240 so I’m a bit too heavy for a lot of round balloons at full inflation so I underinflate a lot. Girlfriend is about 110 pounds and short so we can take something like a Q24 (if I can ever find one again) or a Cattex longneck and I’ll sit on it first to make the neck bulge then she will just sit and ride on that. I’ve also found this way make the balloon last a lot longer than when both of us ride on the bulb because having a girl on the neck keeps a lot of pressure off the knot

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Last two tips I have are put a blanket down on hard floors to keep from popping and make sure your partner (if she’s a girl) pays attention to where her fingernails are wandering, had many a surprise pop because of that.
Rambly drunk post but hope it helps.