I enjoy doing this too. It is difficult to actually stroke her lover's penis while it is in her vagina, but what I have experienced is that the fit is obviously tighter with a couple fingers and the penis in the vagina. Often that extra stimulation excites the guy enough that he comes...then I can easily feel his penis throbbing and flexing as he shoots his semen into her.
Quite often when my young black best buddy is fucking my ex-wife (now his wife) in the missionary position I turn my hand so that its palm is facing upwards, then slowly slip my two longest (2nd and 3rd) fingers into her vagina and with them caress the underbelly of his thrusting cock and the flange of its helmet at the finish of his out-stroke.
That is all erotic enough, but when he finally ejaculates it is always astonishingly copiously such that his freshly spurted semen fills my cupped palm and slithers down onto my wrist. Add to that the ejaculatory jerkings and thumpings of his spurting cock against my two inthrust fingers and my upturned palm, and the erotic impact of his orgasm on me is electrifying , and invariably moves me to up my masturbatory strokings thus triggering my own orgasm.
That said, the most arousing and satisfying aspects of my caressing of my buddy's fully erected bare cock while it is manfully thrusting in my ex's vagina, are firstly my tactilely skin-to-skin sharing with him the breathtakingly beautiful visceral intimacy of his fucking of her, followed by the rhythmical multiple orgasmic contractions of my ex's vaginal milking muscles responding to the powerful ejaculatory jerkings of his cock buried balls-deep in her.
The warm eruption of my buddy's freshly spurted musky semen from between my ex's wide-stretched vaginal rim and the girthy root of my buddy's ejaculating cock, then the slow slithering of the overflow down my wrist and forearm, are my ultimate thrills----especially with me knowing full well that the semen pooling in my palm and running down my forearm has only seconds before been pumped from my buddy's balls and pelvis to be spurted against then bounced back from my ex's battered cervix and the back wall of her maximally stretched-up vagina.
For me there is nothing, but nothing, hotter than knowing that as the beautiful bodies of the fucked-out but still interlocked lovers undergo their die-away orgasmic joltings, the overflow of my buddy's cum-load is still pooling in the palm of my upturned hand, and that I will very soon get to slurp it up and finally savour it.