Actually, last August I decided to get tickle tortured by two dommes at the same time. Long story, but it was a birthday gift to myself!
I had not been tickled by more than one tickler in a very long time, and was curious to see how I would handle it. I made it very clear that I wanted no mercy (which was funny because one of the dommes was actually "named" Mercy) and they were pretty relentless. They came close to getting me to say the safeword, but ran out of time before they could do so.
I can't answer the question about restraints making the tickling more intense or not because I never do tickling without restraints anymore. Last time I did that, I accidentally kicked some poor girl in the head!
The most memorable part of the entire experience for me was twofold, actually: whenever I would focus and concentrate to block out the tickling from one of the dommes, the other would suddenly start tickling a brand new location or say or do something to distract me. I thought that was pretty devious, actually. The second part, of course, was when one of the dommes (jokingly) said she wanted to leave me tied up all afternoon to see how much longer it would take to break me.