Somewhere between. I do plan on sitting back and watching a little. She wants to try two guys so deffentantly go to make that happen for her
The only advice I can give is from my experience. I'm assuming the guy is well hung, experience and good looking. In other words he knows what he's doing. Give him free reign for at least the first 10-15 minutes. If he's skilled he'll have her cumming a few times, not so much as a culmination, but just for her to cut the edge off. 2 or 3 orgasms will make your wife relax considerably. Not just from the orgasms, but she'll be able to relax knowing the guy is skilled.
Plus this gives you a chance to see if he is. Now if he doesn't make her cum maybe that's just not his way, and he's building up to it. Or maybe he's just nervous, or she is just too nervous to cum, in any case I wouldn't pass to a definitive judgment that he wont' work out. But after 15 to 20 minutes if she hasn't cum yet, you may want to step in and play with her a bit. Kiss her, hold her hand, play with her nipples, tell her how beautiful she looks.
If she does cum a few times, you'll have to read her to see if she wants you to step in and join. Some wives are so into the Bull they are annoyed that their husband in distracting them, others want to share in the experience.
In any case when you do step in understand it's like dancing and only one person can be in charge. Unless the other guy is an absolute disaster take your cues from him. Don't share your wife in the 50/50 sense. Share her in the appropriate way of your unique roles. And don't worry when he leaves you'll still be there. You'll have all the time in the world to touch her then.
