I own the Aslan Cherry Jaguar and did a review which I should update. I was able to obtain a "cuff" that is attached to the butt straps and you just insert another dil to this cuff for internal penetration. I use the Tex, for one awesome ride.
I also found that with the low position of the harness, even with one dil, the wide base of a Vixen type dil provides waves of stimulation to my clit while I thrust.
The "cuff" is available from Babeland and also check the Aslan Leather site. The cuff can also be used to secure an anal plug but my butt holds plugs so I haven't tried it that way.
Spend some time making the initial adjustments prior to use...after that it is so easy and comfortable. I hate any interruptions during sex period.