If you've been together 10 years and are a couple of “pervs,” it would seem to me to be a simple act of telling her what you'd like to do.
But since I'm guessing you're not wanting the direct approach, I would suggest starting with the 2 of you having a causal conversation about sexual fantasies, both hers and yours. (And who knows,
You Might get lucky.) Whatever you do, don't start with what you're wanting as the first thing or even the first few things. You want to bury it behind things that are hopefully more outlandish but not too obscene (things like an FFM or MMF 3 way, her being with another girl as you watch, or her getting fucked by another guy as you watch), so that the idea of you wanting to watch her suck or stroke another guy's cock isn't as far out there in comparison. And if you don't already use sex toys in bed, then you'll need to add that little fantasy after the desire to see her suck or stroke another guy. Be careful to not push too hard, and if you have to, keep the discussion as more of a curiosity and for the sake of fun than actually acting on them, but still leaving open the possibility of maybe the 2 of you acting on some of the milder fantasies or fulfilling them with a twist to make them easier to fulfill.
It's this twist that you're looking to exploit if you must, as to start with, you want to ease her into performing a sexual act on another man while you watch by having her do it to a harmless dildo first. If she already has a dildo, then you can ask her to just get it out one night, and if not, you'll need to bring up the discussion of sexual fantasies and express the desire to maybe fulfill some of them, starting with something easy, like adding toys to sex. The point here being to get her to select a dildo to her liking and that she'll be comfortable with sucking, but it would be best if it were more realistic to better simulate the desired end action.
Start with asking her to suck on the dildo as you go down on her, the professed purpose being to lubricate it for when you toy her pussy with it in a few minutes, but the real purpose for you to get her to suck it for your benefit. Don't watch more than the occasional glance, just enjoy the sounds and make sure you give her good oral. After you use the dildo in her pussy and when you move on to having sex, ask her to suck on it while you're fucking her from behind, where you can watch over her shoulder and tell her how much it turns you on (and your actions behind her should also speak to this) and ask her if she's enjoying it (you're wanting her to enjoy it), referring to the dildo as a cock and only as a cock except for when you initially ask her to get it out.
You'll want to do this several times, but not too frequently, and then when she's used to the request and happily does so most if not every time, ask her to do it while you watch because she looks so lovely doing it. As you watch, caress her body and finger her, telling her the whole time how good she looks doing it and how much you're enjoying watching her. The idea here being that you want to make this about her, and not you, so once she's put on the show, make sure you fuck her good while telling her how turned on her act has made you.
Eventually, she should get to the point where she'll want to put on the show for you as a prelude to sex, but if not, move on to asking her to suck the dildo with her facing you so that you can look into each other's eyes, and as you tell her how beautiful she looks sucking the toy (this being the first time you actually refer to the dildo as anything but a cock as she's playing with it), you know she'd look even more gorgeous sucking on a real one and that you would love to see her sucking another guy as you watch, and ask her if she would like to do that.
Since a blowjob is much more intimate than a handjob, and often hand work is involved in a blowjob, you should have no problem getting her to jerk a guy if she'll suck him.
You may also want to use a dildo with a suction base so you can mix things up a bit by sticking it beside you on the shower wall or to the top of a table (or anywhere else it'll stick), where she can go back and forth sucking it and you while jerking the other or jerking you both at the same time.