As an ex-sex toy salesperson, I highly discourage using vaginal toys that simultaneously come in contact with the anus due to the increased risk of vaginal infections and bacterial vaginosis. Also, as a logistical thing, feeldoes are already hard enough to maneuver when you're penetrating a vagina, penetrating an area as tight as the anus must be even harder. I can imagine that in the end, it will just be plain ineffective. On top of that, as a general rule, I discourage the use of any sex toy without a flared base for anal use.

If you're interested in your partner receiving pleasure while using a strap, I recommend using a flat vibrator that can be put inside the harness and curve around comfortably around the pubic area (Like the Cute Little Fuckers Starsi vibe or Fun Factory Laya Spot vibe), or even a C-shaped vibe like the WeVibe Chorus (which is amaaaazing!).